[IPv6] Don't generate temporary for TUN devices

Userland layer-2 tunneling devices allocated through the TUNTAP driver 
(drivers/net/tun.c) have a type of ARPHRD_NONE, and have no link-layer 
address. The kernel complains at regular interval when IPv6 Privacy 
extension are enabled because it can't find an hardware address :

Dec 29 11:02:04 auguste kernel: __ipv6_regen_rndid(idev=cb3e0c00): 
cannot get EUI64 identifier; use random bytes.

IPv6 Privacy extensions should probably be disabled on that sort of 
device. They won't work anyway. If userland wants a more usual 
Ethernet-ish interface with usual IPv6 autoconfiguration, it will use a 
TAP device with an emulated link-layer  and a random hardware address 
rather than a TUN device.

As far as I could fine, TUN virtual device from TUNTAP is the very only 
sort of device using ARPHRD_NONE as kernel device type.

Signed-off-by: Rmi Denis-Courmont <rdenis@simphalempin.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>