bcache: move calculation of stripe_size and io_opt into bcache_device_init

bcache: move calculation of stripe_size and io_opt into bcache_device_init

bcache currently calculates the stripe size for the non-cached_dev
case directly in bcache_device_init, but for the cached_dev case it does
it in the caller.  Consolidate it in one places, which also enables
setting the io_opt queue_limit before allocating the gendisk so that it
can be passed in instead of changing the limit just after the allocation.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20240226104826.283067-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>