generic: test fsync of file with no more hard links

generic: test fsync of file with no more hard links

Test that if we fsync a file that has no more hard links, power fail and
then mount the filesystem, after the journal/log is replayed, the file
doesn't exists anymore.

This exercises a bug recently found and fixed by the following patch for
the linux kernel:

   btrfs: fix fsync of files with no hard links not persisting deletion

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>