nvme-rdma: Check that reinit_request got a proper mr

Warn if req->mr is NULL as it should never happen.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>