generic: add test for fsync after creating hard link

generic: add test for fsync after creating hard link

This test is motivated by an fsync issue discovered in btrfs.
The issue was that we could lose file data, that was previously
fsync'ed successfully, if we end up adding a hard link to our
inode and then persist the fsync log later via an fsync of other
inode for example.

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

  Btrfs: fix fsync data loss after adding hard link to inode

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