btrfs: add test for data corruption when using compression

btrfs: add test for data corruption when using compression

Test for a btrfs data corruption when using compressed
files/extents.  Under certain cases, it was possible for reads to
return random data (content from a previously used page) instead of
zeroes. This also caused partial updates to those regions that were
supposed to be filled with zeroes to save random (and invalid) data
into the file extents.

This is fixed by the commit for the linux kernel titled:

   Btrfs: fix data corruption when reading/updating compressed extents
   (https://patchwork.kernel.org/patch/3610391/)

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>