ip_gre: don't overwrite iflink during net_dev init

iflink is currently set to 0 in __gre_tunnel_init(). This
function is invoked in gre_tap_init() and
ipgre_tunnel_init() which are both used to initialise the
ndo_init field of the respective net_device_ops structs
(ipgre.. and gre_tap..) used by GRE interfaces.

However, in netdevice_register() iflink is first set to -1,
then ndo_init is invoked and then iflink is assigned to a
proper value if and only if it still was -1.

Assigning 0 to iflink in ndo_init is therefore first
preventing netdev_register() to correctly assign it a proper
value and then breaking iflink at all since 0 has not
correct meaning.

Fix this by removing the iflink assignment in
__gre_tunnel_init().

Introduced by c5441932
("GRE: Refactor GRE tunneling code.")
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -660,7 +660,6 @@ static void __gre_tunnel_init(struct net_device *dev)
 
 	dev->needed_headroom	= LL_MAX_HEADER + sizeof(struct iphdr) + 4;
 	dev->mtu		= ETH_DATA_LEN - sizeof(struct iphdr) - 4;
-	dev->iflink		= 0;
 
 	dev->features		|= NETIF_F_NETNS_LOCAL | GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;