nvmet: fix a NULL pointer dereference when tracing the flush command

nvmet: fix a NULL pointer dereference when tracing the flush command

When target side trace in turned on and flush command is issued from the
host it results in the following Oops.

[  856.789724] BUG: kernel NULL pointer dereference, address: 0000000000000068
[  856.790686] #PF: supervisor read access in kernel mode
[  856.791262] #PF: error_code(0x0000) - not-present page
[  856.791863] PGD 6d7110067 P4D 6d7110067 PUD 66f0ad067 PMD 0
[  856.792527] Oops: 0000 [#1] SMP NOPTI
[  856.792950] CPU: 15 PID: 7034 Comm: nvme Tainted: G           OE     5.9.0nvme-5.9+ #71
[  856.793790] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-59-gc9ba5276e3214
[  856.794956] RIP: 0010:trace_event_raw_event_nvmet_req_init+0x13e/0x170 [nvmet]
[  856.795734] Code: 41 5c 41 5d c3 31 d2 31 f6 e8 4e 9b b8 e0 e9 0e ff ff ff 49 8b 55 00 48 8b 38 8b 0
[  856.797740] RSP: 0018:ffffc90001be3a60 EFLAGS: 00010246
[  856.798375] RAX: 0000000000000000 RBX: ffff8887e7d2c01c RCX: 0000000000000000
[  856.799234] RDX: 0000000000000020 RSI: 0000000057e70ea2 RDI: ffff8887e7d2c034
[  856.800088] RBP: ffff88869f710578 R08: ffff888807500d40 R09: 00000000fffffffe
[  856.800951] R10: 0000000064c66670 R11: 00000000ef955201 R12: ffff8887e7d2c034
[  856.801807] R13: ffff88869f7105c8 R14: 0000000000000040 R15: ffff88869f710440
[  856.802667] FS:  00007f6a22bd8780(0000) GS:ffff888813a00000(0000) knlGS:0000000000000000
[  856.803635] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  856.804367] CR2: 0000000000000068 CR3: 00000006d73e0000 CR4: 00000000003506e0
[  856.805283] Call Trace:
[  856.805613]  nvmet_req_init+0x27c/0x480 [nvmet]
[  856.806200]  nvme_loop_queue_rq+0xcb/0x1d0 [nvme_loop]
[  856.806862]  blk_mq_dispatch_rq_list+0x123/0x7b0
[  856.807459]  ? kvm_sched_clock_read+0x14/0x30
[  856.808025]  __blk_mq_sched_dispatch_requests+0xc7/0x170
[  856.808708]  blk_mq_sched_dispatch_requests+0x30/0x60
[  856.809372]  __blk_mq_run_hw_queue+0x70/0x100
[  856.809935]  __blk_mq_delay_run_hw_queue+0x156/0x170
[  856.810574]  blk_mq_run_hw_queue+0x86/0xe0
[  856.811104]  blk_mq_sched_insert_request+0xef/0x160
[  856.811733]  blk_execute_rq+0x69/0xc0
[  856.812212]  ? blk_mq_rq_ctx_init+0xd0/0x230
[  856.812784]  nvme_execute_passthru_rq+0x57/0x130 [nvme_core]
[  856.813461]  nvme_submit_user_cmd+0xeb/0x300 [nvme_core]
[  856.814099]  nvme_user_cmd.isra.82+0x11e/0x1a0 [nvme_core]
[  856.814752]  blkdev_ioctl+0x1dc/0x2c0
[  856.815197]  block_ioctl+0x3f/0x50
[  856.815606]  __x64_sys_ioctl+0x84/0xc0
[  856.816074]  do_syscall_64+0x33/0x40
[  856.816533]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  856.817168] RIP: 0033:0x7f6a222ed107
[  856.817617] Code: 44 00 00 48 8b 05 81 cd 2c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 8
[  856.819901] RSP: 002b:00007ffca848f058 EFLAGS: 00000202 ORIG_RAX: 0000000000000010
[  856.820846] RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00007f6a222ed107
[  856.821726] RDX: 00007ffca848f060 RSI: 00000000c0484e43 RDI: 0000000000000003
[  856.822603] RBP: 0000000000000003 R08: 000000000000003f R09: 0000000000000005
[  856.823478] R10: 00007ffca848ece0 R11: 0000000000000202 R12: 00007ffca84912d3
[  856.824359] R13: 00007ffca848f4d0 R14: 0000000000000002 R15: 000000000067e900
[  856.825236] Modules linked in: nvme_loop(OE) nvmet(OE) nvme_fabrics(OE) null_blk nvme(OE) nvme_corel

Move the nvmet_req_init() tracepoint after we parse the command in
nvmet_req_init() so that we can get rid of the duplicate
nvmet_find_namespace() call.
Rename __assign_disk_name() ->  __assign_req_name(). Now that we call
tracepoint after parsing the command simplify the newly added
__assign_req_name() which fixes this bug.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>