blkcg: clean up blkg_tryget_closest()

The implementation of blkg_tryget_closest() wasn't super obvious and
became a point of suspicion when debugging [1]. So let's clean it up so
it's obviously not the problem.

Also add missing RCU read locking to bio_clone_blkg_association(), which
got exposed by adding the RCU read lock held check in
blkg_tryget_closest().

[1] https://lore.kernel.org/linux-block/a7e97e4b-0dd8-3a54-23b7-a0f27b17fde8@kernel.dk/



Signed-off-by: Dennis Zhou <dennis@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>