nvme-tcp: fix crash triggered with a dataless request submission

write-zeros has a bio, but does not have any data buffers associated
with it. Hence should not initialize the request iter for it (which
attempts to reference the bi_io_vec (and crash).
--
 run blktests nvme/012 at 2021-02-05 21:53:34
 BUG: kernel NULL pointer dereference, address: 0000000000000008
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP NOPTI
 CPU: 15 PID: 12069 Comm: kworker/15:2H Tainted: G S        I       5.11.0-rc6+ #1
 Hardware name: Dell Inc. PowerEdge R640/06NR82, BIOS 2.10.0 11/12/2020
 Workqueue: kblockd blk_mq_run_work_fn
 RIP: 0010:nvme_tcp_init_iter+0x7d/0xd0 [nvme_tcp]
 RSP: 0018:ffffbd084447bd18 EFLAGS: 00010246
 RAX: 0000000000000000 RBX: ffffa0bba9f3ce80 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000002000000
 RBP: ffffa0ba8ac6fec0 R08: 0000000002000000 R09: 0000000000000000
 R10: 0000000002800809 R11: 0000000000000000 R12: 0000000000000000
 R13: ffffa0bba9f3cf90 R14: 0000000000000000 R15: 0000000000000000
 FS:  0000000000000000(0000) GS:ffffa0c9ff9c0000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000008 CR3: 00000001c9c6c005 CR4: 00000000007706e0
 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
 PKRU: 55555554
 Call Trace:
  nvme_tcp_queue_rq+0xef/0x330 [nvme_tcp]
  blk_mq_dispatch_rq_list+0x11c/0x7c0
  ? blk_mq_flush_busy_ctxs+0xf6/0x110
  __blk_mq_sched_dispatch_requests+0x12b/0x170
  blk_mq_sched_dispatch_requests+0x30/0x60
  __blk_mq_run_hw_queue+0x2b/0x60
  process_one_work+0x1cb/0x360
  ? process_one_work+0x360/0x360
  worker_thread+0x30/0x370
  ? process_one_work+0x360/0x360
  kthread+0x116/0x130
  ? kthread_park+0x80/0x80
  ret_from_fork+0x1f/0x30
--

Fixes: cb9b870f

 ("nvme-tcp: fix wrong setting of request iov_iter")
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>