block: floor the discard granularity to the physical block size

Discarding less than a physical block doesn't make sense.  This fixes
the existing behavior for zram before the recent changes to default
the discard granularity to the logical block size, and is also a
generally useful sanity check.

Fixes: 3753039d ("zram: use the default discard granularity")
Reported-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20240103081622.508754-1-hch@lst.de


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