drbd: Consistently use block_id == ID_SYNCER for checksum based resync and online verify

DRBD_MAGIC has nothing to do with block ids and the funny values
computed were not actually used, anyway.

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