btrfs: add regression test for incremental send

btrfs: add regression test for incremental send

Test for a btrfs incremental send issue where we end up sending a
wrong section of data from a file extent if the corresponding file
extent is compressed and the respective file extent item has a non
zero data offset.

Fixed by the following linux kernel btrfs patch:

   Btrfs: use right clone root offset for compressed extents

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>