Replace open-coded encoding logic with the use of conventional XDR
utility functions.

Note that if we replace the cpn_sec and cpn_nsec fields with a
single struct timespec64 field, the encoder can use
nfsd4_encode_nfstime4(), as that is the data type specified by the
XDR spec.

NFS4ERR_INVAL seems inappropriate if the encoder doesn't support
encoding the response. Instead use NFS4ERR_SERVERFAULT, since this
condition is a software bug on the server.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>