nvme-rdma: add helpers for mapping/unmapping request

Introduce nvme_rdma_dma_map_req/nvme_rdma_dma_unmap_req helper functions
to improve code readability and ease on the error flow.

Reviewed-by: Israel Rukshin <israelr@nvidia.com>
Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>