[POWERPC] Fix possible division by zero in scaled time accounting

If we get no user time and no system time allocated since the last
account_system_vtime, the system to user time ratio estimate can end
up dividing by zero.

This was causing a problem noticed by Balbir Singh.
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/time.c

@@ -241,6 +241,7 @@ void account_system_vtime(struct task_struct *tsk)
 		/* deltascaled includes both user and system time.
 		 * Hence scale it based on the purr ratio to estimate
 		 * the system time */
+		if (get_paca()->user_time)
 			deltascaled = deltascaled * get_paca()->system_time /
 			     (get_paca()->system_time + get_paca()->user_time);
 		delta += get_paca()->system_time;