nvmet-rdma: Fix use after free

Avoid dereferencing the queue pointer in nvmet_rdma_release_queue_work()
after it has been freed by nvmet_rdma_free_queue().

Fixes: d8f7750a ("nvmet-rdma: Correctly handle RDMA device hot removal")
Signed-off-by: Vincent Stehlé <vincent.stehle@intel.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>