nvmet-rdma: Fix NULL deref when SEND is completed with error

In the Linux kernel, the following vulnerability has been resolved:

nvmet-rdma: Fix NULL deref when SEND is completed with error

When running some traffic and taking down the link on peer, a
retry counter exceeded error is received. This leads to
nvmet_rdma_error_comp which tried accessing the cq_context to
obtain the queue. The cq_context is no longer valid after the
fix to use shared CQ mechanism and should be obtained similar
to how it is obtained in other functions from the wc->qp.

[ 905.786331] nvmet_rdma: SEND for CQE 0x00000000e3337f90 failed with status transport retry counter exceeded (12).
[ 905.832048] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
[ 905.839919] PGD 0 P4D 0
[ 905.842464] Oops: 0000 1 SMP NOPTI
[ 905.846144] CPU: 13 PID: 1557 Comm: kworker/13:1H Kdump: loaded Tainted: G OE --------- - - 4.18.0-304.el8.x86_64 #1
[ 905.872135] RIP: 0010:nvmet_rdma_error_comp+0x5/0x1b [nvmet_rdma]
[ 905.878259] Code: 19 4f c0 e8 89 b3 a5 f6 e9 5b e0 ff ff 0f b7 75 14 4c 89 ea 48 c7 c7 08 1a 4f c0 e8 71 b3 a5 f6 e9 4b e0 ff ff 0f 1f 44 00 00 <48> 8b 47 48 48 85 c0 74 08 48 89 c7 e9 98 bf 49 00 e9 c3 e3 ff ff
[ 905.897135] RSP: 0018:ffffab601c45fe28 EFLAGS: 00010246
[ 905.902387] RAX: 0000000000000065 RBX: ffff9e729ea2f800 RCX: 0000000000000000
[ 905.909558] RDX: 0000000000000000 RSI: ffff9e72df9567c8 RDI: 0000000000000000
[ 905.916731] RBP: ffff9e729ea2b400 R08: 000000000000074d R09: 0000000000000074
[ 905.923903] R10: 0000000000000000 R11: ffffab601c45fcc0 R12: 0000000000000010
[ 905.931074] R13: 0000000000000000 R14: 0000000000000010 R15: ffff9e729ea2f400
[ 905.938247] FS: 0000000000000000(0000) GS:ffff9e72df940000(0000) knlGS:0000000000000000
[ 905.938249] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 905.950067] nvmet_rdma: SEND for CQE 0x00000000c7356cca failed with status transport retry counter exceeded (12).
[ 905.961855] CR2: 0000000000000048 CR3: 000000678d010004 CR4: 00000000007706e0
[ 905.961855] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 905.961856] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 905.961857] PKRU: 55555554
[ 906.010315] Call Trace:
[ 906.012778] __ib_process_cq+0x89/0x170 [ib_core]
[ 906.017509] ib_cq_poll_work+0x26/0x80 [ib_core]
[ 906.022152] process_one_work+0x1a7/0x360
[ 906.026182] ? create_worker+0x1a0/0x1a0
[ 906.030123] worker_thread+0x30/0x390
[ 906.033802] ? create_worker+0x1a0/0x1a0
[ 906.037744] kthread+0x116/0x130
[ 906.040988] ? kthread_flush_work_fn+0x10/0x10
[ 906.045456] ret_from_fork+0x1f/0x40