ipv6: fix refcnt problem related to POSTDAD state

After running this bonding setup script
    modprobe bonding miimon=100 mode=0 max_bonds=1
    ifconfig bond0 10.1.1.1/16
    ifenslave bond0 eth1
    ifenslave bond0 eth3
on s390 with qeth-driven slaves, modprobe -r fails with this message
    unregister_netdevice: waiting for bond0 to become free. Usage count = 1
due to twice detection of duplicate address.
Problem is caused by a missing decrease of ifp->refcnt in addrconf_dad_failure.
An extra call of in6_ifa_put(ifp) solves it.
Problem has been introduced with commit f2344a1.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -1427,8 +1427,10 @@ void addrconf_dad_failure(struct inet6_ifaddr *ifp)
 {
 	struct inet6_dev *idev = ifp->idev;
 
-	if (addrconf_dad_end(ifp))
+	if (addrconf_dad_end(ifp)) {
+		in6_ifa_put(ifp);
 		return;
+	}
 
 	if (net_ratelimit())
 		printk(KERN_INFO "%s: IPv6 duplicate address %pI6c detected!\n",