loop: always update discard settings in loop_reconfigure_limits

Simplify loop_reconfigure_limits by always updating the discard limits.
This adds a little more work to loop_set_block_size, but doesn't change
the outcome as the discard flag won't change.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240617060532.127975-6-hch@lst.de


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