generic: test for fsync after adding hard links

generic: test for fsync after adding hard links

Test that if we add hard links (in the same directory) to two files and
then fsync only one of the files, after the fsync log/journal is replayed
all the links exist and the filesystem metadata (directory and file
inodes) is in a consistent state.

This test is motivated by a bug found in btrfs that is fixed by linux
kernel patch titled:

  "Btrfs: fix stale directory entries after fsync log replay"

Verified against ext3/4, xfs, f2fs and reiserfs on a 4.1 linux kernel.

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