NFSD: Invoke svc_encode_result_payload() in "read" NFSD encoders

Have the NFSD encoders annotate the boundaries of every
direct-data-placement eligible result data payload. Then change
svcrdma to use that annotation instead of the xdr->page_len
when handling Write chunks.

For NFSv4 on RDMA, that enables the ability to recognize multiple
result payloads per compound. This is a pre-requisite for supporting
multiple Write chunks per RPC transaction.

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