selftests/bpf: fix incorrect users of create_and_get_cgroup

We have some tests that assume create_and_get_cgroup returns -1 on error
which is incorrect (it returns 0 on error). Since fd might be zero in
general case, change create_and_get_cgroup to return -1 on error
and fix the users that assume 0 on error.

Fixes: f269099a7e ("tools/bpf: add a selftest for bpf_get_current_cgroup_id() helper")
Fixes: 7d2c6cfc54 ("bpf: use --cgroup in test_suite if supplied")

v2:
- instead of fixing the uses that assume -1 on error, convert the users
  that assume 0 on error (fd might be zero in general case)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_netcnt.c

@@ -65,7 +65,7 @@ int main(int argc, char **argv)
 
 	/* Create a cgroup, get fd, and join it */
 	cgroup_fd = create_and_get_cgroup(TEST_CGROUP);
-	if (!cgroup_fd) {
+	if (cgroup_fd < 0) {
 		printf("Failed to create test cgroup\n");
 		goto err;
 	}