UPSTREAM: loop: Simplify discard granularity calc

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>
Change-Id: Ia2cc07d3e7af37bce2b0ebd2aa576f72b359eacf
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>
(cherry picked from commit d47de6ac)
Signed-off-by: Bart Van Assche <bvanassche@google.com>