ip6mr: call del_timer_sync() in ip6mr_free_table()

commit 7ba0c47c upstream.

We need to wait for the flying timers, since we
are going to free the mrtable right after it.

Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

net/ipv6/ip6mr.c

@@ -336,7 +336,7 @@ static struct mr6_table *ip6mr_new_table(struct net *net, u32 id)
 
 static void ip6mr_free_table(struct mr6_table *mrt)
 {
-	del_timer(&mrt->ipmr_expire_timer);
+	del_timer_sync(&mrt->ipmr_expire_timer);
 	mroute_clean_tables(mrt, true);
 	kfree(mrt);
 }