proc: don't report kernel addresses in /proc/<pid>/stack

This just changes the file to report them as zero, although maybe even
that could be removed.  I checked, and at least procps doesn't actually
seem to parse the 'stack' file at all.

And since the file doesn't necessarily even exist (it requires
CONFIG_STACKTRACE), possibly other tools don't really use it either.

That said, in case somebody parses it with tools, just having that zero
there should keep such tools happy.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -443,8 +443,7 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
 		save_stack_trace_tsk(task, &trace);
 
 		for (i = 0; i < trace.nr_entries; i++) {
-			seq_printf(m, "[<%pK>] %pB\n",
-				   (void *)entries[i], (void *)entries[i]);
+			seq_printf(m, "[<0>] %pB\n", (void *)entries[i]);
 		}
 		unlock_trace(task);
 	}