NFSD: Add nfsd4_encode_fattr4_change()

Refactor the encoder for FATTR4_CHANGE into a helper. In a
subsequent patch, this helper will be called from a bitmask loop.

The code is restructured a bit to use the modern xdr_stream flow,
and the encoded cinfo value is made const so that callers of the
encoders can be passed a const cinfo.

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