bdi: Export bdi_alloc_node() and bdi_put()

MTD will want to call bdi_alloc_node() and bdi_put() directly. Export
these functions.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>