pidfs: when time ns disabled add check for ioctl

syzbot call pidfd_ioctl() with cmd "PIDFD_GET_TIME_NAMESPACE" and disabled
CONFIG_TIME_NS, since time_ns is NULL, it will make NULL ponter deref in
open_namespace.

Fixes: 5b08bd4 ("pidfs: allow retrieval of namespace file descriptors") # mainline only
Reported-and-tested-by: syzbot+34a0ee986f61f15da35d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=34a0ee986f61f15da35d
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Link: https://lore.kernel.org/r/tencent_7FAE8DB725EE0DD69236DDABDDDE195E4F07@qq.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/pidfs.c

@@ -168,6 +168,8 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	case PIDFD_GET_TIME_NAMESPACE:
 		get_time_ns(nsp->time_ns);
 		ns_common = to_ns_common(nsp->time_ns);
+		if (!nsp->time_ns)
+			return -EINVAL;
 		break;
 	case PIDFD_GET_TIME_FOR_CHILDREN_NAMESPACE:
 		get_time_ns(nsp->time_ns_for_children);