fstests: generic test for directory fsync after adding hard links

fstests: generic test for directory fsync after adding hard links

This test is motivated by an fsync issue discovered in btrfs.
The issue was that after adding a new hard link to an existing file
(one that was created in a past transaction) and fsync'ing the parent
directory of the new hard link, after the fsync log replay the file's
inode link count did not get its link count incremented, while the new
directory entry was visible.
Also, unlike xfs and ext4, new files under the directory we fsync were
not being written to the fsync log, nor were any child directories and
new files and links under the children directories. So this test verifies
too that btrfs has the same behaviour as xfs and ext4.

The btrfs issue was fixed by the following linux kernel patch:

  Btrfs: fix metadata inconsistencies after directory fsync

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>