Add function that registers bdi and takes va_list instead of variable
number of arguments.

Add bdi_alloc() as simple wrapper for NUMA-unaware users allocating BDI.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NJens Axboe <axboe@fb.com>