btrfs: simple sparse copy testcase for btrfs

btrfs: simple sparse copy testcase for btrfs

Tests file clone functionality of btrfs ("reflinks"):
   - Reflink a file
   - Reflink the reflinked file
   - Modify the original file
   - Modify the reflinked file

[sandeen: add helpers, make several mostly-cosmetic
 changes to the original testcase]

Signed-off-by: Koen De Wit <koen.de.wit@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>