procfs: fix /proc/<pid>/stat stack pointer for kernel threads

Fix a small issue for the stack pointer in /proc/<pid>/stat.  In case of a
kernel thread the value of the printed stack pointer should be 0.
Signed-off-by: Stefani Seibold <stefani@seibold.net>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/array.c

@@ -571,7 +571,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		rsslim,
 		mm ? mm->start_code : 0,
 		mm ? mm->end_code : 0,
-		(permitted) ? task->stack_start : 0,
+		(permitted && mm) ? task->stack_start : 0,
 		esp,
 		eip,
 		/* The signal information here is obsolete.