ARM: fix /proc/$PID/stack on SMP

Rabin Vincent reports:
| On SMP, this BUG() in save_stack_trace_tsk() can be easily triggered
| from user space by reading /proc/$PID/stack, where $PID is any pid but
| the current process:
|
|	if (tsk != current) {
| #ifdef CONFIG_SMP
|		/*
|		 * What guarantees do we have here that 'tsk'
|		 * is not running on another CPU?
|		 */
|		BUG();
| #else

Fix this by replacing the BUG() with an entry to terminate the stack
trace, returning an empty trace - I'd rather not expose the dwarf
unwinder to a volatile stack of a running thread.
Reported-by: Rabin Vincent <rabin@rab.in>
Tested-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/stacktrace.c

@@ -94,10 +94,13 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 	if (tsk != current) {
 #ifdef CONFIG_SMP
 		/*
-		 * What guarantees do we have here that 'tsk'
-		 * is not running on another CPU?
+		 * What guarantees do we have here that 'tsk' is not
+		 * running on another CPU?  For now, ignore it as we
+		 * can't guarantee we won't explode.
 		 */
-		BUG();
+		if (trace->nr_entries < trace->max_entries)
+			trace->entries[trace->nr_entries++] = ULONG_MAX;
+		return;
 #else
 		data.no_sched_functions = 1;
 		frame.fp = thread_saved_fp(tsk);