[$] Adding BPF to blk-iocost

The scheduling of block I/O requests has longbeen a challenge for operating-system kernels. For many years, theperformance characteristics of rotating drives meant that puttingconsiderable resources into request ordering was worthwhile. In a worldwith fast, solid-state drives, scheduling is more concerned with enforcingfairness between competing users while being fast enough to keep up withdrives that can perform millions of I/Ooperations per second. The blk-iocost I/Ocontroller was designed for the solid-state world and generally performswell, but there is always a desire to do better. This patchseries from Tao Cui aims to make blk-iocost more flexible by enablingthe loading of a BPF program to make cost decisions.