Merge branch 'Provide NULL and KERNEL_VERSION macros in bpf_helpers.h'

Andrii Nakryiko says:

====================

Provide NULL and KERNEL_VERSION macros in bpf_helpers.h. Patch #2 removes such
custom NULL definition from one of the selftests.

v2->v3:
  - instead of vmlinux.h, do this in bpf_helpers.h;
  - added KERNEL_VERSION, which comes up periodically as well;
  - I dropped strict compilation patches for now, because we run into new
    warnings (e.g., not checking read() result) in kernel-patches CI, which
    I can't even reproduce locally. Also -Wdiscarded-qualifiers pragma for
    jit_disasm.c is not supported by Clang, it needs to be
    -Wincompatible-pointer-types-discards-qualifiers for Clang; we don't have
    to deal with that in this patch set;
v1->v2:
  - fix few typos and wrong copy/paste;
  - fix #pragma push -> pop.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>