block: have drivers use blk_queue_max_discard_sectors()

block: have drivers use blk_queue_max_discard_sectors()

Some drivers use it now, others just set the limits field manually.
But in preparation for splitting this into a hard and soft limit,
ensure that they all call the proper function for setting the hw
limit for discards.

Signed-off-by: Jens Axboe <axboe@fb.com>