fs: add directories hash collision test

This tests creates several directories that have the same small (8)
group of hashes to ensure the hash ordering of file and directories
are preserved.

Sample backtrace this test tries to prevent in future:

[ 3856.245843] XFS (vda1): Internal error xfs_trans_cancel at line 966 of file fs/xfs/xfs_trans.c.  Caller 0xffffffffa01186bc
[ 3856.249049] CPU: 1 PID: 866 Comm: rm Not tainted 3.13.6-200.fc20.x86_64 #1
[ 3856.250966] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 3856.252615]  000000000000000c ffff8800d23a7d68 ffffffff8168730c ffff8800cf5462b8
[ 3856.254823]  ffff8800d23a7d80 ffffffffa00d00cb ffffffffa01186bc ffff8800d23a7da8
[ 3856.257241]  ffffffffa00e5459 ffff8800d9ac3400 ffff8800d23a7e30 ffff8800371b6800
[ 3856.259420] Call Trace:
[ 3856.260172]  [<ffffffff8168730c>] dump_stack+0x45/0x56
[ 3856.261717]  [<ffffffffa00d00cb>] xfs_error_report+0x3b/0x40 [xfs]
[ 3856.263472]  [<ffffffffa01186bc>] ? xfs_remove+0x1ac/0x370 [xfs]
[ 3856.270838]  [<ffffffffa00e5459>] xfs_trans_cancel+0xd9/0x100 [xfs]
[ 3856.272783]  [<ffffffffa01186bc>] xfs_remove+0x1ac/0x370 [xfs]
[ 3856.274531]  [<ffffffffa00db40b>] xfs_vn_unlink+0x4b/0x90 [xfs]
[ 3856.276286]  [<ffffffff811c61b8>] vfs_rmdir+0xa8/0x100
[ 3856.277821]  [<ffffffff811c638d>] do_rmdir+0x17d/0x1d0
[ 3856.281021]  [<ffffffff811ba7fe>] ? ____fput+0xe/0x10
[ 3856.285261]  [<ffffffff8108c11c>] ? task_work_run+0xac/0xe0
[ 3856.286952]  [<ffffffff81013a31>] ? do_notify_resume+0x61/0xa0
[ 3856.288693]  [<ffffffff811c9a65>] SyS_unlinkat+0x25/0x40
[ 3856.290407]  [<ffffffff816962e9>] system_call_fastpath+0x16/0x1b
[ 3856.292685] XFS (vda1): xfs_do_force_shutdown(0x8) called from line 967 of file fs/xfs/xfs_trans.c.  Return address = 0xffffffffa00e5472
[ 3856.627330] XFS (vda1): Corruption of in-memory data detected.  Shutting down filesystem
[ 3856.627332] XFS (vda1): Please umount the filesystem and rectify the problem(s)

With help from Mark Tinguely, thanks very much!

[dchinner: changed binary/test file name to dirhash_collide[.c]]

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Tested-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/group

@@ -3,6 +3,7 @@
 003 db auto quick
 004 db auto quick
 005 auto quick
+006 auto dir
 008 rw ioctl auto quick
 009 rw ioctl auto prealloc quick
 012 rw auto quick