[$] Yet another try at the BPF program allocator

The BPF subsystem, which allows code to be loaded into the kernel from userspace and safely executed in the kernel context, is bound to create a number ofchallenges for the kernel as a whole. One might not think that allocatingmemory for BPF programs would be high on the list of problems, but life(and memory management) can be surprising. The attempts to do a better jobof providing space for compiled BPF code have, to date, only been partiallysuccessful; now Song Liu is back with a newapproach to finish the job.