UPSTREAM: loop: Use bdev limit helpers for configuring discard

Instead of directly looking at the request_queue limits, use the bdev
limits helpers, which is preferable.

Change-Id: Ia4eca604290e7cdb4dcd0057ab6d2b4aa1d7d973
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241030111900.3981223-1-john.g.garry@oracle.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 8d3fd059)
Signed-off-by: Bart Van Assche <bvanassche@google.com>