selftests/bpf: Add missing prototypes for several test kfuncs

Adding missing prototypes for several kfuncs that are used by
test_verifier tests. We don't really need kfunc prototypes for
these tests, but adding them to silence 'make W=1' build and
to have all test kfuncs declarations in bpf_testmod_kfunc.h.

Also moving __diag_pop for -Wmissing-prototypes to cover also
bpf_testmod_test_write and bpf_testmod_test_read and adding
bpf_fentry_shadow_test in there as well. All of them need to
be exported, but there's no need for declarations.

Fixes: 65eb006d ("bpf: Move kernel test kfuncs to bpf_testmod")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Closes: https://lore.kernel.org/oe-kbuild-all/202306051319.EihCQZPs-lkp@intel.com
Link: https://lore.kernel.org/bpf/20230607224046.236510-1-jolsa@kernel.org