Fixed a regression in resync decission code drbd_uuid_compare() [Bugz 260]

Since 8.3.3 we fail to do the resync when a partial resynch is not
possible, but a full synch is necessary.

This regression was introduced with 7101539930c0a89146959e7a39c09ad9c3516434

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>