nvme-rdma: do not try to stop unallocated queues

Trying to stop a queue which hasn't been allocated will result
in a warning due to calling mutex_lock() against an uninitialized mutex.

 DEBUG_LOCKS_WARN_ON(lock->magic != lock)
 WARNING: CPU: 4 PID: 104150 at kernel/locking/mutex.c:579

 Call trace:
  RIP: 0010:__mutex_lock+0x1173/0x14a0
  nvme_rdma_stop_queue+0x1b/0xa0 [nvme_rdma]
  nvme_rdma_teardown_io_queues.part.0+0xb0/0x1d0 [nvme_rdma]
  nvme_rdma_delete_ctrl+0x50/0x100 [nvme_rdma]
  nvme_do_delete_ctrl+0x149/0x158 [nvme_core]

Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>