block: fix bio_split_rw_at to take zone_write_granularity into account

Otherwise it can create unaligned writes on zoned devices.

Fixes: a805a4fa ("block: introduce zone_write_granularity limit")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20241104062647.91160-3-hch@lst.de


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