BPF uapi structures and 32-bit

[David Miller <davem@davemloft.net>]

In the linux/bpf.h UAPI header, we must absolutely avoid any
non-fixed-sized types.

Otherwise we have serious problems on 32-bit.

Unfortunately I discovered today that we have take on two such cases,
sk_msg_md and sk_reuseport_md, both of which start with two void
pointers.

I hit this because test_verifier.c on my sparc64 box was built as
a 32-bit binary and this causes a hundred or so tests to fail, and
many if not all are because of the changing struct layout.

I could built 64-bit but this absolutely should work properly.

But for fully native 32-bit it is even worse, this will really
need to be resolved because llvm/clang is always going to build
the BPF programs with 64-bit void pointers.

I would strongly suggest we try and fix this now if we can.