selftests/bpf: Fix bogus uninitialized variable warning

When compiling selftests/bpf in optimized mode (-O2), GCC erroneously
complains about uninitialized token variable:

  In file included from network_helpers.c:22:
  network_helpers.c: In function ‘open_netns’:
  test_progs.h:355:22: error: ‘token’ may be used uninitialized [-Werror=maybe-uninitialized]
    355 |         int ___err = libbpf_get_error(___res);                          \
        |                      ^~~~~~~~~~~~~~~~~~~~~~~~
  network_helpers.c:440:14: note: in expansion of macro ‘ASSERT_OK_PTR’
    440 |         if (!ASSERT_OK_PTR(token, "malloc token"))
        |              ^~~~~~~~~~~~~
  In file included from /data/users/andriin/linux/tools/testing/selftests/bpf/tools/include/bpf/libbpf.h:21,
                   from bpf_util.h:9,
                   from network_helpers.c:20:
  /data/users/andriin/linux/tools/testing/selftests/bpf/tools/include/bpf/libbpf_legacy.h:113:17: note: by argument 1 of type ‘const void *’ to ‘libbpf_get_error’ declared here
    113 | LIBBPF_API long libbpf_get_error(const void *ptr);
        |                 ^~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  make: *** [Makefile:522: /data/users/andriin/linux/tools/testing/selftests/bpf/network_helpers.o] Error 1

This is completely bogus becuase libbpf_get_error() doesn't dereference
pointer, but the only easy way to silence this is to allocate initialized
memory with calloc().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20220705224818.4026623-2-andrii@kernel.org