bpf: Selftests, sockmap test prog run without setting cgroup

Running test_sockmap with arguments to specify a test pattern requires
including a cgroup argument. Instead of requiring this if the option is
not provided create one

This is not used by selftest runs but I use it when I want to test a
specific test. Most useful when developing new code and/or tests.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/158939722675.15176.6294210959489131688.stgit@john-Precision-5820-Tower