selftests/bpf: Adjust vmtest.sh to use local kernel configuration

So far the vmtest.sh script, which can be used as a convenient way to
run bpf selftests, has obtained the kernel config safe to use for
testing from the libbpf/libbpf GitHub repository [0].

Given that we now have included this configuration into this very
repository, we can just consume it from here as well, eliminating the
necessity of remote accesses.

With this change we adjust the logic in the script to use the
configuration from below tools/testing/selftests/bpf/configs/ instead
of pulling it over the network.

  [0] https://github.com/libbpf/libbpf



Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Mykola Lysenko <mykolal@fb.com>
Link: https://lore.kernel.org/bpf/20220727001156.3553701-4-deso@posteo.net