sd: factor out a sd_discard_mode helper

Split the logic to pick the right discard mode into a little helper
to prepare for further changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Link: https://lore.kernel.org/r/20240531074837.1648501-10-hch@lst.de


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