[PATCH 3/3] block: introduce the BFQ-v6 I/O sched for 3.0

Add the BFQ-v6 I/O scheduler to 3.0.
The general structure is borrowed from CFQ, as much code. A (bfq_)queue is
associated to each task doing I/O on a device, and each time a scheduling
decision has to be made a queue is selected and served until it expires.