drbd: refactor the backing dev max_segments calculation

Factor out a drbd_backing_dev_max_segments helper that checks the
backing device limitation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Philipp Reisner <philipp.reisner@linbit.com>
Reviewed-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Tested-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20240306140332.623759-4-philipp.reisner@linbit.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>