Similar to commit 38f88c45 ("bonding/alb: properly access headers
in bond_alb_xmit()"), we need to make sure arp header was pulled
in skb->head before blindly accessing it in rlb_arp_xmit().

Remove arp_pkt() private helper, since it is more readable/obvious
to have the following construct back to back :

	if (!pskb_network_may_pull(skb, sizeof(*arp)))
		return NULL;
	arp = (struct arp_pkt *)skb_network_header(skb);

syzbot reported :

BUG: KMSAN: uninit-value in bond_slave_has_mac_rx include/net/bonding.h:704 [inline]
BUG: KMSAN: uninit-value in rlb_arp_xmit drivers/net/bonding/bond_alb.c:662 [inline]
BUG: KMSAN: uninit-value in bond_alb_xmit+0x575/0x25e0 drivers/net/bonding/bond_alb.c:1477
CPU: 0 PID: 12743 Comm: syz-executor.4 Not tainted 5.6.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 bond_slave_has_mac_rx include/net/bonding.h:704 [inline]
 rlb_arp_xmit drivers/net/bonding/bond_alb.c:662 [inline]
 bond_alb_xmit+0x575/0x25e0 drivers/net/bonding/bond_alb.c:1477
 __bond_start_xmit drivers/net/bonding/bond_main.c:4257 [inline]
 bond_start_xmit+0x85d/0x2f70 drivers/net/bonding/bond_main.c:4282
 __netdev_start_xmit include/linux/netdevice.h:4524 [inline]
 netdev_start_xmit include/linux/netdevice.h:4538 [inline]
 xmit_one net/core/dev.c:3470 [inline]
 dev_hard_start_xmit+0x531/0xab0 net/core/dev.c:3486
 __dev_queue_xmit+0x37de/0x4220 net/core/dev.c:4063
 dev_queue_xmit+0x4b/0x60 net/core/dev.c:4096
 packet_snd net/packet/af_packet.c:2967 [inline]
 packet_sendmsg+0x8347/0x93b0 net/packet/af_packet.c:2992
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 __sys_sendto+0xc1b/0xc50 net/socket.c:1998
 __do_sys_sendto net/socket.c:2010 [inline]
 __se_sys_sendto+0x107/0x130 net/socket.c:2006
 __x64_sys_sendto+0x6e/0x90 net/socket.c:2006
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45c479
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fc77ffbbc78 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: ffffffffffffffda RBX: 00007fc77ffbc6d4 RCX: 000000000045c479
RDX: 000000000000000e RSI: 00000000200004c0 RDI: 0000000000000003
RBP: 000000000076bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000a04 R14: 00000000004cc7b0 R15: 000000000076bf2c

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:144 [inline]
 kmsan_internal_poison_shadow+0x66/0xd0 mm/kmsan/kmsan.c:127
 kmsan_slab_alloc+0x8a/0xe0 mm/kmsan/kmsan_hooks.c:82
 slab_alloc_node mm/slub.c:2793 [inline]
 __kmalloc_node_track_caller+0xb40/0x1200 mm/slub.c:4401
 __kmalloc_reserve net/core/skbuff.c:142 [inline]
 __alloc_skb+0x2fd/0xac0 net/core/skbuff.c:210
 alloc_skb include/linux/skbuff.h:1051 [inline]
 alloc_skb_with_frags+0x18c/0xa70 net/core/skbuff.c:5766
 sock_alloc_send_pskb+0xada/0xc60 net/core/sock.c:2242
 packet_alloc_skb net/packet/af_packet.c:2815 [inline]
 packet_snd net/packet/af_packet.c:2910 [inline]
 packet_sendmsg+0x66a0/0x93b0 net/packet/af_packet.c:2992
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg net/socket.c:672 [inline]
 __sys_sendto+0xc1b/0xc50 net/socket.c:1998
 __do_sys_sendto net/socket.c:2010 [inline]
 __se_sys_sendto+0x107/0x130 net/socket.c:2006
 __x64_sys_sendto+0x6e/0x90 net/socket.c:2006
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Jay Vosburgh <j.vosburgh@gmail.com>
Cc: Veaceslav Falico <vfalico@gmail.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>