tty: Fix slave peer index accounting

We need to test every even index (which represents a slave
peer) thus use modulo operation here.
Reported-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tty.c

@@ -167,7 +167,7 @@ int tty_verify_active_pairs(void)
 	unsigned long i, unpaired_slaves = 0;
 
 	for_each_bit(i, tty_active_pairs) {
-		if (is_log2(i)) {
+		if ((i % 2) == 0) {
 			if (test_bit(i + 1, tty_active_pairs)) {
 				i++;
 				continue;