generic: add test for fsync of directory after creating hard link

Test that if we create a new hard link for a file which was
previously fsync'ed, fsync a parent directory of the new hard link
and power fail, the parent directory exists after mounting the
filesystem again. The parent directory must be a new directory, not
yet persisted.

This test is motivated by a bug found in btrfs, where the fsync'ed
parent directory was lost after a power failure. The bug in btrfs is
fixed by a patch for the linux kernel titled:

 "Btrfs: sync log after logging new name"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/group

@@ -500,3 +500,4 @@
 495 auto quick swap
 496 auto quick swap
 497 auto quick swap collapse
+498 auto quick log