drbd: resync: fix too large bursts for very slow rates

While merging adjacent dirty blocks into resync requests,
the resync rate throttle was disregarded.
For very low resync rates, the effective rate may have exceeded
the intended rate by a larger margin.

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