ipvs: Verify that IP_VS protocol has been registered

The registration of a protocol might fail, there were no checks
and all registrations were assumed to be correct. This lead to
NULL ptr dereferences when apps tried registering.

For example:

[ 1293.226051] BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
[ 1293.227038] IP: [<ffffffff822aacb0>] tcp_register_app+0x60/0xb0
[ 1293.227038] PGD 391de067 PUD 6c20b067 PMD 0
[ 1293.227038] Oops: 0000 [#1] PREEMPT SMP
[ 1293.227038] CPU 1
[ 1293.227038] Pid: 19609, comm: trinity Tainted: G        W    3.4.0-rc1-next-20120405-sasha-dirty #57
[ 1293.227038] RIP: 0010:[<ffffffff822aacb0>]  [<ffffffff822aacb0>] tcp_register_app+0x60/0xb0
[ 1293.227038] RSP: 0018:ffff880038c1dd18  EFLAGS: 00010286
[ 1293.227038] RAX: ffffffffffffffc0 RBX: 0000000000001500 RCX: 0000000000010000
[ 1293.227038] RDX: 0000000000000000 RSI: ffff88003a2d5888 RDI: 0000000000000282
[ 1293.227038] RBP: ffff880038c1dd48 R08: 0000000000000000 R09: 0000000000000000
[ 1293.227038] R10: 0000000000000000 R11: 0000000000000000 R12: ffff88003a2d5668
[ 1293.227038] R13: ffff88003a2d5988 R14: ffff8800696a8ff8 R15: 0000000000000000
[ 1293.227038] FS:  00007f01930d9700(0000) GS:ffff88007ce00000(0000) knlGS:0000000000000000
[ 1293.227038] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 1293.227038] CR2: 0000000000000018 CR3: 0000000065dfc000 CR4: 00000000000406e0
[ 1293.227038] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 1293.227038] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[ 1293.227038] Process trinity (pid: 19609, threadinfo ffff880038c1c000, task ffff88002dc73000)
[ 1293.227038] Stack:
[ 1293.227038]  ffff880038c1dd48 00000000fffffff4 ffff8800696aada0 ffff8800694f5580
[ 1293.227038]  ffffffff8369f1e0 0000000000001500 ffff880038c1dd98 ffffffff822a716b
[ 1293.227038]  0000000000000000 ffff8800696a8ff8 0000000000000015 ffff8800694f5580
[ 1293.227038] Call Trace:
[ 1293.227038]  [<ffffffff822a716b>] ip_vs_app_inc_new+0xdb/0x180
[ 1293.227038]  [<ffffffff822a7258>] register_ip_vs_app_inc+0x48/0x70
[ 1293.227038]  [<ffffffff822b2fea>] __ip_vs_ftp_init+0xba/0x140
[ 1293.227038]  [<ffffffff821c9060>] ops_init+0x80/0x90
[ 1293.227038]  [<ffffffff821c90cb>] setup_net+0x5b/0xe0
[ 1293.227038]  [<ffffffff821c9416>] copy_net_ns+0x76/0x100
[ 1293.227038]  [<ffffffff810dc92b>] create_new_namespaces+0xfb/0x190
[ 1293.227038]  [<ffffffff810dca21>] unshare_nsproxy_namespaces+0x61/0x80
[ 1293.227038]  [<ffffffff810afd1f>] sys_unshare+0xff/0x290
[ 1293.227038]  [<ffffffff8187622e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
[ 1293.227038]  [<ffffffff82665539>] system_call_fastpath+0x16/0x1b
[ 1293.227038] Code: 89 c7 e8 34 91 3b 00 89 de 66 c1 ee 04 31 de 83 e6 0f 48 83 c6 22 48 c1 e6 04 4a 8b 14 26 49 8d 34 34 48 8d 42 c0 48 39 d6 74 13 <66> 39 58 58 74 22 48 8b 48 40 48 8d 41 c0 48 39 ce 75 ed 49 8d
[ 1293.227038] RIP  [<ffffffff822aacb0>] tcp_register_app+0x60/0xb0
[ 1293.227038]  RSP <ffff880038c1dd18>
[ 1293.227038] CR2: 0000000000000018
[ 1293.379284] ---[ end trace 364ab40c7011a009 ]---
[ 1293.381182] Kernel panic - not syncing: Fatal exception in interrupt

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>