Merge branch 'bpf: Move kernel test kfuncs into bpf_testmod'

Jiri Olsa says:

====================
hi,
I noticed several times in discussions that we should move test kfuncs
into kernel module, now perhaps even more pressing with all the kfunc
effort. This patchset moves all the test kfuncs into bpf_testmod.

I added bpf_testmod/bpf_testmod_kfunc.h header that is shared between
bpf_testmod kernel module and BPF programs.

v4 changes:
  - s390 supports long calls [1] now, so it can call now kfuncs from module [Ilya]
  - added acks [David]
  - cleanups for ptr_to_u64 function [David]
  - use relative path for bpf_testmod_kfunc.h include [Andrii]
  - new libbpf fix (patch 1) for gen_loader

v3 changes:
  - added acks [David]
  - added bpf_testmod.ko make dependency for bpf test progs [David]
  - better handling of __ksym and refcount_t in bpf_testmod_kfunc.h [David]
  - removed 'extern' from kfuncs declarations [David]
  - typo in header guard macro [David]
  - use only stdout in un/load_bpf_testmod

v2 changes:
  - add 74bc3a5a into bpf-next/master CI, so the test would pass
    https://github.com/kernel-patches/vmtest/pull/192
  - remove extra externs [Artem]
  - using un/load_bpf_testmod in other tests
  - rebased

thanks,
jirka

[1] 1cf3bfc6

 bpf: Support 64-bit pointers to kfuncs
====================

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