bdi: convert bdi_debug_register to int

Convert bdi_debug_register to int and then do error handle for it.

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