loop: Simplify discard granularity calc

[ Upstream commit d47de6ac ]

A bdev discard granularity is always at least SECTOR_SIZE, so don't check
for a zero value.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20241101092215.422428-1-john.g.garry@oracle.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: f5c84eff ("loop: Add sanity check for read/write_iter")
Signed-off-by: Sasha Levin <sashal@kernel.org>