NFSD: Add nfsd4_encode_fattr4_mounted_on_fileid()

Refactor the encoder for FATTR4_MOUNTED_ON_FILEID into a helper. In
a subsequent patch, this helper will be called from a bitmask loop.

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