btrfs: read corruption of compressed extents

btrfs: read corruption of compressed extents

Regression test for file read corruption when using compressed extents
that are shared by multiple consecutive ranges of the same file.

The btrfs issue is fixed by the linux kernel patch titled:
"Btrfs: fix read corruption of compressed and shared extents"

Without the corresponding fix the test fails because the second time it
reads the test files it gets different data (some pages are incorrectly
filled with zeroes) from the data it wrote before doing a clean ummount.

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