bcache: share register sysfs with async register

Previously the experimental async registration uses a separate sysfs
file register_async. Now the async registration code seems working well
for a while, we can do furtuher testing with it now.

This patch changes the async bcache registration shares the same sysfs
file /sys/fs/bcache/register (and register_quiet). Async registration
will be default behavior if BCACHE_ASYNC_REGISTRATION is set in kernel
configure. By default, BCACHE_ASYNC_REGISTRATION is not configured yet.

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