svcrdma: Add svc_rdma_map_reply_hdr()

Introduce a helper to DMA-map a reply's transport header before
sending it. This will in part replace the map vector cache.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>