btrfs: add test for btrfs incremental send data corruption

btrfs: add test for btrfs incremental send data corruption

Regression test for the btrfs incremental send feature, where the kernel
would incorrectly consider a range of a file as a hole and send a stream
of 0 bytes to the destination (send stream) that would overwrite the
corresponding file region.

This issue is fixed by the following linux kernel btrfs patch:

   Btrfs: send, fix data corruption due to incorrect hole detection
   (https://patchwork.kernel.org/patch/3910081/)

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>