btrfs: sparse copy of a directory tree on btrfs

btrfs: sparse copy of a directory tree on btrfs

Tests file clone functionality of btrfs ("reflinks") on directory trees.
   - Create directory and subdirectory, each having one file
   - Create 2 recursive reflinked copies of the tree
   - Modify the original files
   - Modify one of the copies

[sandeen: mostly cosmetic changes]

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>