generic: test fsync of a file truncated to a length of zero

generic: test fsync of a file truncated to a length of zero

Test that if we truncate a file to 0, fsync it, add a hard link to the
file and then fsync the parent directory, after a power failure the file
has a size of 0 (and the hardlink exists too).

This exercises a bug fixed by the following kernel patch for btrfs:

  "btrfs: fix zero size inode with non-zero size after log replay"

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>