xfs: Regression test for vulnerable directory integrity check

If a malicious XFS contains a block+ format directory wherein the
directory inode's core.mode is corrupted, and there are
subdirectories of the corrupted directory, an attempt to traverse up
the directory tree by running xfs_scrub will crash the kernel in
__xfs_dir3_data_check.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>