samples/bpf: ensure ipv6 is enabled before running tests

samples/bpf: ensure ipv6 is enabled before running tests

Currently, a few of BPF tests use ipv6 functionality. The problem here
is that if ipv6 is disabled, these tests will fail, and even if the
test fails, it will not tell you why it failed.

    $ sudo ./test_cgrp2_sock2.sh
    RTNETLINK answers: Permission denied

In order to fix this, this commit ensures ipv6 is enabled prior to
running tests.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20230115071613.125791-2-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>