btrfs: test for incremental send after file extent cloning

btrfs: test for incremental send after file extent cloning

Test that an incremental send works after a file gets one of its extents
cloned/deduplicated into lower file offsets.

This is a regression test for the problem fixed by the linux kernel patch
titled:

  "Btrfs: teach backref walking about backrefs with underflowed
   offset values"

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