NFSD: Clean up nfsd4_encode_copy()

Restructure this function using conventional XDR utility functions
and so it aligns better with the XDR in the specification.

I've also moved nfsd4_encode_copy() closer to the data type encoders
that only it uses.

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