Btrfs: Test code to change the order of delayed-ref processing

Normally delayed refs get processed in ascending bytenr order. This
correlates in most cases to the order added. To expose dependencies
on this order, we start to process the tree in the middle instead of
the beginning.
This code is only effective when SCRAMBLE_DELAYED_REFS is defined.

Signed-off-by: Arne Jansen <sensille@gmx.net>