svcrdma: Refactor the RDMA Write path

Refactor for subsequent changes.

Constify the xdr_buf argument to ensure the code here does not
modify it, and to enable callers to pass in a
"const struct xdr_buf *".

At the same time, rename the helper functions, which emit RDMA
Writes, not RDMA Sends, and add documenting comments.

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