[$] A BPF-specific memory allocator

The kernel does not lack for memory allocators, so one might well questionthe need for yet another one. As thispatch set from Alexei Starovoitov makes clear, though, the BPFsubsystem feels such a need. The proposed new allocator is intended toincrease the reliability of allocations made within BPF programs, which mightbe run in just about any execution context.