bcache: move bucket related code into read_super_common()

Setting sb->first_bucket and checking sb->keys indeed are only for cache
device, it does not make sense to do them in read_super() for backing
device too.

This patch moves the related code piece into read_super_common()
explicitly for cache device and avoid the confusion.

Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>