igmp: fix incorrect unsolicit report count when join group

We should not start timer if im->unsolicit_count equal to 0 after decrease.
Or we will send one more unsolicit report message. i.e. 3 instead of 2 by
default.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: NHangbin Liu <liuhangbin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -820,10 +820,9 @@ static void igmp_timer_expire(struct timer_list *t)
 	spin_lock(&im->lock);
 	im->tm_running = 0;
 
-	if (im->unsolicit_count) {
-		im->unsolicit_count--;
+	if (im->unsolicit_count && --im->unsolicit_count)
 		igmp_start_timer(im, unsolicited_report_interval(in_dev));
-	}
+
 	im->reporter = 1;
 	spin_unlock(&im->lock);