[POWERPC] Use is_init() instead of pid==1

Use is_init() rather than hard coded pid comparison.

Cc: Anton Blanchard <anton@samba.org>
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/traps.c

@@ -174,7 +174,7 @@ void _exception(int signr, struct pt_regs *regs, int code, unsigned long addr)
 	 * generate the same exception over and over again and we get
 	 * nowhere.  Better to kill it and let the kernel panic.
 	 */
-	if (current->pid == 1) {
+	if (is_init(current)) {
 		__sighandler_t handler;
 
 		spin_lock_irq(&current->sighand->siglock);