block: add WARN_ON if bdi register fail

device_add_disk need do more safety error handle, so this patch just
add WARN_ON.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>

Adapted for current series by me.
Signed-off-by: Jens Axboe <axboe@kernel.dk>