drbd: bm_page_async_io: fix spurious bitmap "IO error" on large volumes

drbd: bm_page_async_io: fix spurious bitmap "IO error" on large volumes

We usually do all our bitmap IO in units of PAGE_SIZE.

With very small or oddly sized external meta data, or with
PAGE_SIZE != 4k, it can happen that our last on-disk bitmap page
is not fully PAGE_SIZE aligned, so we may need to adjust the size
of the IO.

We used to do that with
  min_t(unsigned int, PAGE_SIZE,
	last_allowed_sector - current_offset);
And for just the right diff, (unsigned int)(diff) will result in 0.

A bio of length 0 will correctly be rejected with an IO error
(and some scary WARN_ON_ONCE()) by the scsi layer.

Do the calculation properly.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20220622204932.196830-1-christoph.boehmwalder@linbit.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>