ip6_pol_route panic: Do not allow VLAN on loopback

Several tests in the ipv6 routing code check IFF_LOOPBACK, and
allowing stacking such as VLAN'ing on top of loopback results in a
netdevice which reports IFF_LOOPBACK but really isn't the loopback
device.

Instead of spamming the ipv6 routing code with even more special tests,
simply disallow VLAN over loopback.

The result of this patch is:

# modprobe 8021q
# vconfig add lo 43
ERROR: trying to add VLAN #43 to IF -:lo:- error: Operation not supported

Signed-off-by: Krishna Kumar <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/loopback.c

···
 		| NETIF_F_RXCSUM
 		| NETIF_F_HIGHDMA
 		| NETIF_F_LLTX
-		| NETIF_F_NETNS_LOCAL;
+		| NETIF_F_NETNS_LOCAL
+		| NETIF_F_VLAN_CHALLENGED;
 	dev->ethtool_ops	= &loopback_ethtool_ops;
 	dev->header_ops		= &eth_header_ops;
 	dev->netdev_ops		= &loopback_ops;