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>