samples: make pidfd-metadata fail gracefully on older kernels

Initialize pidfd to an invalid descriptor, to fail gracefully on
those kernels that do not implement CLONE_PIDFD and leave pidfd
unchanged.
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Christian Brauner <christian@brauner.io>

samples/pidfd/pidfd-metadata.c

@@ -83,7 +83,7 @@ static int pidfd_metadata_fd(pid_t pid, int pidfd)
 
 int main(int argc, char *argv[])
 {
-	int pidfd = 0, ret = EXIT_FAILURE;
+	int pidfd = -1, ret = EXIT_FAILURE;
 	char buf[4096] = { 0 };
 	pid_t pid;
 	int procfd, statusfd;
@@ -91,7 +91,11 @@ int main(int argc, char *argv[])
 
 	pid = pidfd_clone(CLONE_PIDFD, &pidfd);
 	if (pid < 0)
-		exit(ret);
+		err(ret, "CLONE_PIDFD");
+	if (pidfd == -1) {
+		warnx("CLONE_PIDFD is not supported by the kernel");
+		goto out;
+	}
 
 	procfd = pidfd_metadata_fd(pid, pidfd);
 	close(pidfd);