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: ad568218 ("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>