nvme-rdma: factor out a nvme_rdma_end_request helper

Factor a small sniplet of duplicated code into a new helper in
preparation for making this sniplet a little bit less trivial.

Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>