ipv6: Fix Smatch warning.

With commit d6ddef9e(IPv6: Fix not join all-router mcast group
when forwarding set.) I check 'dev' after it's dereference that
leads to a Smatch complaint:

net/ipv6/addrconf.c:438 ipv6_add_dev()
	 warn: variable dereferenced before check 'dev' (see line 432)

net/ipv6/addrconf.c
   431		/* protected by rtnl_lock */
   432		rcu_assign_pointer(dev->ip6_ptr, ndev);
                                   ^^^^^^^^^^^^
Old dereference.

   433
   434		/* Join all-node multicast group */
   435		ipv6_dev_mc_inc(dev, &in6addr_linklocal_allnodes);
   436
   437		/* Join all-router multicast group if forwarding is set
*/
   438		if (ndev->cnf.forwarding && dev && (dev->flags &
IFF_MULTICAST))
                                            ^^^

Remove the check to avoid the complaint as 'dev' can't be NULL.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NLi Wei <lw@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -435,7 +435,7 @@ static struct inet6_dev * ipv6_add_dev(struct net_device *dev)
 	ipv6_dev_mc_inc(dev, &in6addr_linklocal_allnodes);
 
 	/* Join all-router multicast group if forwarding is set */
-	if (ndev->cnf.forwarding && dev && (dev->flags & IFF_MULTICAST))
+	if (ndev->cnf.forwarding && (dev->flags & IFF_MULTICAST))
 		ipv6_dev_mc_inc(dev, &in6addr_linklocal_allrouters);
 
 	return ndev;