[XFS] Fix assertion in xfs_attr_shortform_remove().

SGI-PV: 960791
SGI-Modid: xfs-linux-melb:xfs-kern:28021a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>