svcrdma: Update synopsis of svc_rdma_map_reply_msg()

Preparing for subsequent patches, no behavior change expected.

Pass the RPC Call's svc_rdma_recv_ctxt deeper into the sendto()
path. This enables passing more information about Requester-
provided Write and Reply chunks into those lower-level functions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>