drbd: Update some outdated comments to match the code

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

diff --cc drivers/block/drbd/drbd_bitmap.c
Simple merge

diff --cc drivers/block/drbd/drbd_nl.c
Simple merge

diff --cc drivers/block/drbd/drbd_req.c
Simple merge

diff --cc drivers/block/drbd/drbd_worker.c
Simple merge