tools/bpf: add a selftest for bpf_get_current_cgroup_id() helper

Syscall name_to_handle_at() can be used to get cgroup id
for a particular cgroup path in user space. The selftest
got cgroup id from both user and kernel, and compare to
ensure they are equal to each other.

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/cgroup_helpers.h

@@ -13,5 +13,6 @@ int create_and_get_cgroup(char *path);
 int join_cgroup(char *path);
 int setup_cgroup_environment(void);
 void cleanup_cgroup_environment(void);
+unsigned long long get_cgroup_id(char *path);
 
 #endif