block: add a blk_alloc_discard_bio helper

Factor out a helper from __blkdev_issue_discard that chews off as much as
possible from a discard range and allocates a bio for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240506042027.2289826-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2 files changed