block: remove bio_disassociate_blkg

bio_disassociate_blkg has two callers, of which one immediately assigns
a new value to >bi_blkg.  Just open code the function in the two callers.

Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>