[$] A memory allocator for BPF code

Loading a BPF program into the kernel involves a lot of steps, includingverification, permissions checking, linking to in-kernel helper functions,and compilation to the native instruction format. Underneath all of that,though, lies one other simple task: allocating some memory to store thecompiled BPF program in the kernel's address space. It turns out that thisallocation can be somewhat wasteful of memory in current kernels, especially onsystems where large numbers of BPF programs are loaded. Thispatch set from Song Liu seeks to remedy this problem by introducing yetanother specialized memory allocator into the kernel.