xfstests: add test for btrfs incremental send data corruption

xfstests: add test for btrfs incremental send data corruption

Btrfs incremental send had an issue where it would detect a non-existent
file hole and then overwrite the file section that hole covers with zeroes,
overriding file data that it shouldn't.

The respective btrfs kernel patch that fixed this issue is titled:

   Btrfs: fix send file hole detection leading to data corruption
   (https://patchwork.kernel.org/patch/3544831/)

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>