nvmet-tcp: fix kernel crash if commands allocation fails
In the Linux kernel, the following vulnerability has been resolved:

nvmet-tcp: fix kernel crash if commands allocation fails

If the commands allocation fails in nvmet_tcp_alloc_cmds()
the kernel crashes in nvmet_tcp_release_queue_work() because of
a NULL pointer dereference.

  nvmet: failed to install queue 0 cntlid 1 ret 6
  Unable to handle kernel NULL pointer dereference at
         virtual address 0000000000000008

Fix the bug by setting queue->nr_cmds to zero in case
nvmet_tcp_alloc_cmd() fails.