block, bfq: fix uaf for bfqq in bic_set_bfqq()

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

block, bfq: fix uaf for bfqq in bic_set_bfqq()

After commit 64dc8c732f5c ("block, bfq: fix possible uaf for 'bfqq->bic'"),

bic->bfqq will be accessed in bic_set_bfqq(), however, in some context

bic->bfqq will be freed, and bic_set_bfqq() is called with the freed

bic->bfqq.

Fix the problem by always freeing bfqq after bic_set_bfqq().