sysrq: Properly check for kernel threads

There's a real possibility of killing kernel threads that might
have issued use_mm(), so kthread's mm might become non-NULL.

This patch fixes the issue by checking for PF_KTHREAD (just as
get_task_mm()).

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/sysrq.c

@@ -324,9 +324,12 @@ static void send_sig_all(int sig)
 
 	read_lock(&tasklist_lock);
 	for_each_process(p) {
-		if (p->mm && !is_global_init(p))
-			/* Not swapper, init nor kernel thread */
-			force_sig(sig, p);
+		if (p->flags & PF_KTHREAD)
+			continue;
+		if (is_global_init(p))
+			continue;
+
+		force_sig(sig, p);
 	}
 	read_unlock(&tasklist_lock);
 }