xfs: xfs_attr_shortform_allfit() does not handle attr3 format.

xfstests generic/117 fails with:

XFS: Assertion failed: leaf->hdr.info.magic == cpu_to_be16(XFS_ATTR_LEAF_MAGIC)

indicating a function that does not handle the attr3 format
correctly. Fix it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>