nvme: fix multipath crash caused by flush request when blktrace is enabled

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

nvme: fix multipath crash caused by flush request when blktrace is enabled

The flush request initialized by blk_kick_flush has NULL bio,
and it may be dealt with nvme_end_req during io completion.
When blktrace is enabled, nvme_trace_bio_complete with multipath
activated trying to access NULL pointer bio from flush request
results in the following crash:

[ 2517.831677] BUG: kernel NULL pointer dereference, address: 000000000000001a
[ 2517.835213] #PF: supervisor read access in kernel mode
[ 2517.838724] #PF: error_code(0x0000) - not-present page
[ 2517.842222] PGD 7b2d51067 P4D 0
[ 2517.845684] Oops: 0000 [#1] SMP NOPTI
[ 2517.849125] CPU: 2 PID: 732 Comm: kworker/2:1H Kdump: loaded Tainted: G S 5.15.67-0.cl9.x86_64 #1
[ 2517.852723] Hardware name: XFUSION 2288H V6/BC13MBSBC, BIOS 1.13 07/27/2022
[ 2517.856358] Workqueue: nvme_tcp_wq nvme_tcp_io_work [nvme_tcp]
[ 2517.859993] RIP: 0010:blk_add_trace_bio_complete+0x6/0x30
[ 2517.863628] Code: 1f 44 00 00 48 8b 46 08 31 c9 ba 04 00 10 00 48 8b 80 50 03 00 00 48 8b 78 50 e9 e5 fe ff ff 0f 1f 44 00 00 41 54 49 89 f4 55 <0f> b6 7a 1a 48 89 d5 e8 3e 1c 2b 00 48 89 ee 4c 89 e7 5d 89 c1 ba
[ 2517.871269] RSP: 0018:ff7f6a008d9dbcd0 EFLAGS: 00010286
[ 2517.875081] RAX: ff3d5b4be00b1d50 RBX: 0000000002040002 RCX: ff3d5b0a270f2000
[ 2517.878966] RDX: 0000000000000000 RSI: ff3d5b0b021fb9f8 RDI: 0000000000000000
[ 2517.882849] RBP: ff3d5b0b96a6fa00 R08: 0000000000000001 R09: 0000000000000000
[ 2517.886718] R10: 000000000000000c R11: 000000000000000c R12: ff3d5b0b021fb9f8
[ 2517.890575] R13: 0000000002000000 R14: ff3d5b0b021fb1b0 R15: 0000000000000018
[ 2517.894434] FS: 0000000000000000(0000) GS:ff3d5b42bfc80000(0000) knlGS:0000000000000000
[ 2517.898299] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2517.902157] CR2: 000000000000001a CR3: 00000004f023e005 CR4: 0000000000771ee0
[ 2517.906053] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2517.909930] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 2517.913761] PKRU: 55555554
[ 2517.917558] Call Trace:
[ 2517.921294] <TASK>
[ 2517.924982] nvme_complete_rq+0x1c3/0x1e0 [nvme_core]
[ 2517.928715] nvme_tcp_recv_pdu+0x4d7/0x540 [nvme_tcp]
[ 2517.932442] nvme_tcp_recv_skb+0x4f/0x240 [nvme_tcp]
[ 2517.936137] ? nvme_tcp_recv_pdu+0x540/0x540 [nvme_tcp]
[ 2517.939830] tcp_read_sock+0x9c/0x260
[ 2517.943486] nvme_tcp_try_recv+0x65/0xa0 [nvme_tcp]
[ 2517.947173] nvme_tcp_io_work+0x64/0x90 [nvme_tcp]
[ 2517.950834] process_one_work+0x1e8/0x390
[ 2517.954473] worker_thread+0x53/0x3c0
[ 2517.958069] ? process_one_work+0x390/0x390
[ 2517.961655] kthread+0x10c/0x130
[ 2517.965211] ? set_kthread_struct+0x40/0x40
[ 2517.968760] ret_from_fork+0x1f/0x30
[ 2517.972285] </TASK>

To avoid this situation, add a NULL check for req->bio before
calling trace_block_bio_complete.