NFSD: Clean up nfs4svc_encode_compoundres()

In today's Linux NFS server implementation, the NFS dispatcher
initializes each XDR result stream, and the NFSv4 .pc_func and
.pc_encode methods all use xdr_stream-based encoding. This keeps
rq_res.len automatically updated. There is no longer a need for
the WARN_ON_ONCE() check in nfs4svc_encode_compoundres().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>