drbd: fix al-to-on-disk-bitmap for 4k logical_block_size

Up to now, applying the in-core activity-log to the on-disk
bitmap did not care for logical_block_size.

On logical_block_size != 512 byte, this very likely results
in misalligned block access and spurious "io errors".

We now simply always submit aligned whole 4k blocks, fixing this
for logical block sizes of 512, 1024, 2048 and 4096.

For even larger logical block sizes, this won't work.
But I'm not aware of devices with such properties being available.

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