if the IPVS module is removed while the sync daemon is starting, there is
a small gap where try_module_get() might fail getting the refcount inside
ip_vs_use_count_inc(). Then, the refcounts of IPVS module are unbalanced,
and the subsequent call to stop_sync_thread() causes the following splat:

 WARNING: CPU: 0 PID: 4013 at kernel/module.c:1146 module_put.part.44+0x15b/0x290
  Modules linked in: ip_vs(-) nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 veth ip6table_filter ip6_tables iptable_filter binfmt_misc intel_rapl_msr intel_rapl_common crct10dif_pclmul crc32_pclmul ext4 mbcache jbd2 ghash_clmulni_intel snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_intel_nhlt snd_hda_codec snd_hda_core snd_hwdep snd_seq snd_seq_device snd_pcm aesni_intel crypto_simd cryptd glue_helper joydev pcspkr snd_timer virtio_balloon snd soundcore i2c_piix4 nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi virtio_net net_failover virtio_blk failover virtio_console qxl drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ata_piix ttm crc32c_intel serio_raw drm virtio_pci libata virtio_ring virtio floppy dm_mirror dm_region_hash dm_log dm_mod [last unloaded: nf_defrag_ipv6]
  CPU: 0 PID: 4013 Comm: modprobe Tainted: G        W         5.4.0-rc1.upstream+ #741
  Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
  RIP: 0010:module_put.part.44+0x15b/0x290
  Code: 04 25 28 00 00 00 0f 85 18 01 00 00 48 83 c4 68 5b 5d 41 5c 41 5d 41 5e 41 5f c3 89 44 24 28 83 e8 01 89 c5 0f 89 57 ff ff ff <0f> 0b e9 78 ff ff ff 65 8b 1d 67 83 26 4a 89 db be 08 00 00 00 48
  RSP: 0018:ffff888050607c78 EFLAGS: 00010297
  RAX: 0000000000000003 RBX: ffffffffc1420590 RCX: ffffffffb5db0ef9
  RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffffffffc1420590
  RBP: 00000000ffffffff R08: fffffbfff82840b3 R09: fffffbfff82840b3
  R10: 0000000000000001 R11: fffffbfff82840b2 R12: 1ffff1100a0c0f90
  R13: ffffffffc1420200 R14: ffff88804f533300 R15: ffff88804f533ca0
  FS:  00007f8ea9720740(0000) GS:ffff888053800000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007f3245abe000 CR3: 000000004c28a006 CR4: 00000000001606f0
  Call Trace:
   stop_sync_thread+0x3a3/0x7c0 [ip_vs]
   ip_vs_sync_net_cleanup+0x13/0x50 [ip_vs]
   ops_exit_list.isra.5+0x94/0x140
   unregister_pernet_operations+0x29d/0x460
   unregister_pernet_device+0x26/0x60
   ip_vs_cleanup+0x11/0x38 [ip_vs]
   __x64_sys_delete_module+0x2d5/0x400
   do_syscall_64+0xa5/0x4e0
   entry_SYSCALL_64_after_hwframe+0x49/0xbe
  RIP: 0033:0x7f8ea8bf0db7
  Code: 73 01 c3 48 8b 0d b9 80 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 b0 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 89 80 2c 00 f7 d8 64 89 01 48
  RSP: 002b:00007ffcd38d2fe8 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
  RAX: ffffffffffffffda RBX: 0000000002436240 RCX: 00007f8ea8bf0db7
  RDX: 0000000000000000 RSI: 0000000000000800 RDI: 00000000024362a8
  RBP: 0000000000000000 R08: 00007f8ea8eba060 R09: 00007f8ea8c658a0
  R10: 00007ffcd38d2a60 R11: 0000000000000206 R12: 0000000000000000
  R13: 0000000000000001 R14: 00000000024362a8 R15: 0000000000000000
  irq event stamp: 4538
  hardirqs last  enabled at (4537): [<ffffffffb6193dde>] quarantine_put+0x9e/0x170
  hardirqs last disabled at (4538): [<ffffffffb5a0556a>] trace_hardirqs_off_thunk+0x1a/0x20
  softirqs last  enabled at (4522): [<ffffffffb6f8ebe9>] sk_common_release+0x169/0x2d0
  softirqs last disabled at (4520): [<ffffffffb6f8eb3e>] sk_common_release+0xbe/0x2d0

Check the return value of ip_vs_use_count_inc() and let its caller return
proper error. Inside do_ip_vs_set_ctl() the module is already refcounted,
we don't need refcount/derefcount there. Finally, in register_ip_vs_app()
and start_sync_thread(), take the module refcount earlier and ensure it's
released in the error path.

Change since v1:
 - better return values in case of failure of ip_vs_use_count_inc(),
   thanks to Julian Anastasov
 - no need to increase/decrease the module refcount in ip_vs_set_ctl(),
   thanks to Julian Anastasov
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>