drbd: split out a drbd_discard_supported helper

Add a helper to check if discard is supported for a given connection /
backing device combination.

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/20240306140332.623759-7-philipp.reisner@linbit.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>