Refactor the encoder for FATTR4_TIME_DELTA into a helper. In a
subsequent patch, this helper will be called from a bitmask loop.

fattr4_time_delta is specified as an nfstime4, so de-duplicate this
encoder.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>