Fix attempt to a possible use-after-free of queues

[Arianna Avanzini]

Hello,

we have spotted a possible use-after-free of data structures used by BFQ (namely, of bfq_queues). The attached patches should fix the issue; it should be possible to apply them cleanly on BFQ-v6 and the specified kernel versions and build the resulting code without warnings.

While in our tests the problem did not cause crashes, it may result in an erratic behavior of the SLUB allocator: it is therefore recommended to apply the proposed fix.

Please let us know about any issues you may encounter.