Refactor the encoder for FATTR4_FILEHANDLE into a helper. In a
subsequent patch, this helper will be called from a bitmask loop.

We can de-duplicate the other filehandle encoder (in GETFH) using
our new helper.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>