nvme-rdma: fix crash due to incorrect cqe

A crash happened due to injecting error test.
When a CQE has incorrect command id due do an error injection, the host
may find a request which is already freed.  Dereferencing req->mr->rkey
causes a crash in nvme_rdma_process_nvme_rsp because the mr is already
freed.

Add a check for the mr to fix it.

Signed-off-by: Chao Leng <lengchao@huawei.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>