selftests/seccomp: Use __NR_mknodat instead of __NR_mknod

The __NR_mknod syscall doesn't exist on arm64 (only __NR_mknodat).
Switch to the modern syscall.

Fixes: ad56821 ("selftests/seccomp: Check for EPOLLHUP for user_notif")
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-16-keescook@chromium.org
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -3746,7 +3746,7 @@ TEST(user_notification_filter_empty)
 	if (pid == 0) {
 		int listener;
 
-		listener = user_notif_syscall(__NR_mknod, SECCOMP_FILTER_FLAG_NEW_LISTENER);
+		listener = user_notif_syscall(__NR_mknodat, SECCOMP_FILTER_FLAG_NEW_LISTENER);
 		if (listener < 0)
 			_exit(EXIT_FAILURE);