drbd: use blk_queue_max_discard_sectors helper

We currently only set q->limits.max_discard_sectors, but that is not
enough. Another field, max_hw_discard_sectors, was introduced in
commit 0034af03 ("block: make /sys/block/<dev>/queue/discard_max_bytes
writeable").

The difference is that max_discard_sectors can be changed from user
space via sysfs, while max_hw_discard_sectors is the "hardware" upper
limit.

So use this helper, which sets both.

This is also a fixup for commit 998e9cbc ("drbd: cleanup
decide_on_discard_support"): if discards are not supported, that does
not necessarily mean we also want to disable write_zeroes.

Fixes: 998e9cbc ("drbd: cleanup decide_on_discard_support")
Reviewed-by: Joel Colledge <joel.colledge@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Link: https://lore.kernel.org/r/20221109133453.51652-2-christoph.boehmwalder@linbit.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_nl.c

@@ -1210,6 +1210,7 @@ static void decide_on_discard_support(struct drbd_device *device,
 	struct drbd_connection *connection =
 		first_peer_device(device)->connection;
 	struct request_queue *q = device->rq_queue;
+	unsigned int max_discard_sectors;
 
 	if (bdev && !bdev_max_discard_sectors(bdev->backing_bdev))
 		goto not_supported;
@@ -1230,15 +1231,14 @@ static void decide_on_discard_support(struct drbd_device *device,
 	 * topology on all peers.
 	 */
 	blk_queue_discard_granularity(q, 512);
-	q->limits.max_discard_sectors = drbd_max_discard_sectors(connection);
-	q->limits.max_write_zeroes_sectors =
-		drbd_max_discard_sectors(connection);
+	max_discard_sectors = drbd_max_discard_sectors(connection);
+	blk_queue_max_discard_sectors(q, max_discard_sectors);
+	blk_queue_max_write_zeroes_sectors(q, max_discard_sectors);
 	return;
 
 not_supported:
 	blk_queue_discard_granularity(q, 0);
-	q->limits.max_discard_sectors = 0;
-	q->limits.max_write_zeroes_sectors = 0;
+	blk_queue_max_discard_sectors(q, 0);
 }
 
 static void fixup_write_zeroes(struct drbd_device *device, struct request_queue *q)