bfq: Make sure bfqg for which we are queueing requests is online

In the Linux kernel, the following vulnerability has been resolved:

bfq: Make sure bfqg for which we are queueing requests is online

Bios queued into BFQ IO scheduler can be associated with a cgroup that

was already offlined. This may then cause insertion of this bfq_group

into a service tree. But this bfq_group will get freed as soon as last

bio associated with it is completed leading to use after free issues for

service tree users. Fix the problem by making sure we always operate on

online bfq_group. If the bfq_group associated with the bio is not

online, we pick the first online parent.