After recent refactoring of block offlads infrastructure, indr_dev->block
pointer is dereferenced before it is verified to be non-NULL. Example stack
trace where this behavior leads to NULL-pointer dereference error when
creating vxlan dev on system with mlx5 NIC with offloads enabled:

[ 1157.852938] ==================================================================
[ 1157.866877] BUG: KASAN: null-ptr-deref in tc_indr_block_ing_cmd.isra.41+0x9c/0x160
[ 1157.880877] Read of size 4 at addr 0000000000000090 by task ip/3829
[ 1157.901637] CPU: 22 PID: 3829 Comm: ip Not tainted 5.2.0-rc6+ #488
[ 1157.914438] Hardware name: Supermicro SYS-2028TP-DECR/X10DRT-P, BIOS 2.0b 03/30/2017
[ 1157.929031] Call Trace:
[ 1157.938318]  dump_stack+0x9a/0xeb
[ 1157.948362]  ? tc_indr_block_ing_cmd.isra.41+0x9c/0x160
[ 1157.960262]  ? tc_indr_block_ing_cmd.isra.41+0x9c/0x160
[ 1157.972082]  __kasan_report+0x176/0x192
[ 1157.982513]  ? tc_indr_block_ing_cmd.isra.41+0x9c/0x160
[ 1157.994348]  kasan_report+0xe/0x20
[ 1158.004324]  tc_indr_block_ing_cmd.isra.41+0x9c/0x160
[ 1158.015950]  ? tcf_block_setup+0x430/0x430
[ 1158.026558]  ? kasan_unpoison_shadow+0x30/0x40
[ 1158.037464]  __tc_indr_block_cb_register+0x5f5/0xf20
[ 1158.049288]  ? mlx5e_rep_indr_tc_block_unbind+0xa0/0xa0 [mlx5_core]
[ 1158.062344]  ? tc_indr_block_dev_put.part.47+0x5c0/0x5c0
[ 1158.074498]  ? rdma_roce_rescan_device+0x20/0x20 [ib_core]
[ 1158.086580]  ? br_device_event+0x98/0x480 [bridge]
[ 1158.097870]  ? strcmp+0x30/0x50
[ 1158.107578]  mlx5e_nic_rep_netdevice_event+0xdd/0x180 [mlx5_core]
[ 1158.120212]  notifier_call_chain+0x6d/0xa0
[ 1158.130753]  register_netdevice+0x6fc/0x7e0
[ 1158.141322]  ? netdev_change_features+0xa0/0xa0
[ 1158.152218]  ? vxlan_config_apply+0x210/0x310 [vxlan]
[ 1158.163593]  __vxlan_dev_create+0x2ad/0x520 [vxlan]
[ 1158.174770]  ? vxlan_changelink+0x490/0x490 [vxlan]
[ 1158.185870]  ? rcu_read_unlock+0x60/0x60 [vxlan]
[ 1158.196798]  vxlan_newlink+0x99/0xf0 [vxlan]
[ 1158.207303]  ? __vxlan_dev_create+0x520/0x520 [vxlan]
[ 1158.218601]  ? rtnl_create_link+0x3d0/0x450
[ 1158.228900]  __rtnl_newlink+0x8a7/0xb00
[ 1158.238701]  ? stack_access_ok+0x35/0x80
[ 1158.248450]  ? rtnl_link_unregister+0x1a0/0x1a0
[ 1158.258735]  ? find_held_lock+0x6d/0xd0
[ 1158.268379]  ? is_bpf_text_address+0x67/0xf0
[ 1158.278330]  ? lock_acquire+0xc1/0x1f0
[ 1158.287686]  ? is_bpf_text_address+0x5/0xf0
[ 1158.297449]  ? is_bpf_text_address+0x86/0xf0
[ 1158.307310]  ? kernel_text_address+0xec/0x100
[ 1158.317155]  ? arch_stack_walk+0x92/0xe0
[ 1158.326497]  ? __kernel_text_address+0xe/0x30
[ 1158.336213]  ? unwind_get_return_address+0x2f/0x50
[ 1158.346267]  ? create_prof_cpu_mask+0x20/0x20
[ 1158.355936]  ? arch_stack_walk+0x92/0xe0
[ 1158.365117]  ? stack_trace_save+0x8a/0xb0
[ 1158.374272]  ? stack_trace_consume_entry+0x80/0x80
[ 1158.384226]  ? match_held_lock+0x33/0x210
[ 1158.393216]  ? kasan_unpoison_shadow+0x30/0x40
[ 1158.402593]  rtnl_newlink+0x53/0x80
[ 1158.410925]  rtnetlink_rcv_msg+0x3a5/0x600
[ 1158.419777]  ? validate_linkmsg+0x400/0x400
[ 1158.428620]  ? find_held_lock+0x6d/0xd0
[ 1158.437117]  ? match_held_lock+0x1b/0x210
[ 1158.445760]  ? validate_linkmsg+0x400/0x400
[ 1158.454642]  netlink_rcv_skb+0xc7/0x1f0
[ 1158.463150]  ? netlink_ack+0x470/0x470
[ 1158.471538]  ? netlink_deliver_tap+0x1f3/0x5a0
[ 1158.480607]  netlink_unicast+0x2ae/0x350
[ 1158.489099]  ? netlink_attachskb+0x340/0x340
[ 1158.497935]  ? _copy_from_iter_full+0xde/0x3b0
[ 1158.506945]  ? __virt_addr_valid+0xb6/0xf0
[ 1158.515578]  ? __check_object_size+0x159/0x240
[ 1158.524515]  netlink_sendmsg+0x4d3/0x630
[ 1158.532879]  ? netlink_unicast+0x350/0x350
[ 1158.541400]  ? netlink_unicast+0x350/0x350
[ 1158.549805]  sock_sendmsg+0x94/0xa0
[ 1158.557561]  ___sys_sendmsg+0x49d/0x570
[ 1158.565625]  ? copy_msghdr_from_user+0x210/0x210
[ 1158.574457]  ? __fput+0x1e2/0x330
[ 1158.581948]  ? __kasan_slab_free+0x130/0x180
[ 1158.590407]  ? kmem_cache_free+0xb6/0x2d0
[ 1158.598574]  ? mark_lock+0xc7/0x790
[ 1158.606177]  ? task_work_run+0xcf/0x100
[ 1158.614165]  ? exit_to_usermode_loop+0x102/0x110
[ 1158.622954]  ? __lock_acquire+0x963/0x1ee0
[ 1158.631199]  ? lockdep_hardirqs_on+0x260/0x260
[ 1158.639777]  ? match_held_lock+0x1b/0x210
[ 1158.647918]  ? lockdep_hardirqs_on+0x260/0x260
[ 1158.656501]  ? match_held_lock+0x1b/0x210
[ 1158.664643]  ? __fget_light+0xa6/0xe0
[ 1158.672423]  ? __sys_sendmsg+0xd2/0x150
[ 1158.680334]  __sys_sendmsg+0xd2/0x150
[ 1158.688063]  ? __ia32_sys_shutdown+0x30/0x30
[ 1158.696435]  ? lock_downgrade+0x2e0/0x2e0
[ 1158.704541]  ? mark_held_locks+0x1a/0x90
[ 1158.712611]  ? mark_held_locks+0x1a/0x90
[ 1158.720619]  ? do_syscall_64+0x1e/0x2c0
[ 1158.728530]  do_syscall_64+0x78/0x2c0
[ 1158.736254]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 1158.745414] RIP: 0033:0x7f62d505cb87
[ 1158.753070] Code: 64 89 02 48 c7 c0 ff ff ff ff eb b9 0f 1f 80 00 00 00 00 8b 05 6a 2b 2c 00 48 63 d2 48 63 ff 85 c0 75 18 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 59 f3 c3 0f 1f 80 00 00[87/1817]
 48 89 f3 48
[ 1158.780924] RSP: 002b:00007fffd9832268 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[ 1158.793204] RAX: ffffffffffffffda RBX: 000000005d26048f RCX: 00007f62d505cb87
[ 1158.805111] RDX: 0000000000000000 RSI: 00007fffd98322d0 RDI: 0000000000000003
[ 1158.817055] RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000006
[ 1158.828987] R10: 00007f62d50ce260 R11: 0000000000000246 R12: 0000000000000001
[ 1158.840909] R13: 000000000067e540 R14: 0000000000000000 R15: 000000000067ed20
[ 1158.852873] ==================================================================

Introduce new function tcf_block_non_null_shared() that verifies block
pointer before dereferencing it to obtain index. Use the function in
tc_indr_block_ing_cmd() to prevent NULL pointer dereference.

Fixes: 955bcb6e ("drivers: net: use flow block API")
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>