[XFS] don't encode parent in nfs filehandles unless nessecary

As Dave pointed out after the export ops changes we now always encode the
parent into the filehandle for regular files, but it's not actually needed
when the filesystem is export with no_subtree_check. This one-liner fixes
xfs_fs_encode_fh to skip encoding the parent unless nessecary.

SGI-PV: 976035
SGI-Modid: xfs-linux-melb:xfs-kern:30535a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>