generic: test for fsync after adding hard link to a file

generic: test for fsync after adding hard link to a file

This test is motivated by an issue found in btrfs.

It tests that after syncing the filesystem, adding a hard link to a file,
syncing the filesystem again, doing a write to the file that increases
its size and then doing a fsync against that file, durably persists the
data written to the file. That is, after log/journal replay, the data
is available.

The btrfs issue is fixed by the commit titled:

  "Btrfs: fix fsync data loss after append write"

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