[$] A BPF-specific memory allocator

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