block: really clone the block cgroup in bio_clone_blkg_association

bio_clone_blkg_association is supposed to clone the associatation, but
actually ends up doing a search with a tryget.  As we know we have a
reference on the source cgroup just get an unconditional additional
reference to it and call it a day.  That also removes the need for
a RCU critical section.

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>