perfcounters/powerpc: fix oops with multiple counters in a group

Impact: fix oops-causing bug

This fixes a bug in the powerpc hw_perf_counter_init where the code
didn't initialize ctrs[n] before passing the ctrs array to check_excludes,
leading to possible oopses and other incorrect behaviour.  This fixes it
by initializing ctrs[n] correctly.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/perf_counter.c

@@ -633,6 +633,7 @@ hw_perf_counter_init(struct perf_counter *counter)
 			return NULL;
 	}
 	events[n] = ev;
+	ctrs[n] = counter;
 	if (check_excludes(ctrs, n, 1))
 		return NULL;
 	if (power_check_constraints(events, n + 1))