drbd: get rid of atomic update on disk bitmap works

Just trigger the occasional lazy bitmap write-out during resync
from the central wait_for_work() helper.

Previously, during resync, bitmap pages would be written out separately,
synchronously, one at a time, at least 8 times each (every 512 bytes
worth of bitmap cleared).

Now we trigger "merge friendly" bulk write out of all cleared pages
every two seconds during resync, and once the resync is finished.
Most pages will be written out only once.

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