generic: add test for fsync after cloning file range

generic: add test for fsync after cloning file range

Test that if we do a buffered write to a file, fsync it, clone a
range from another file into our file that overlaps the previously
written range, fsync the file again and then power fail, after we
mount again the filesystem, no file data was lost or corrupted.

This test is motivated by a bug found in btrfs, which is fixed by a
patch for the linux kernel titled:

  "Btrfs: fix file data corruption after cloning a range and fsync"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>