ip_gre: fix IFLA_MTU ignored on NEWLINK

It's safe to remove the setting of dev's needed_headroom and mtu in
__gre_tunnel_init, as discussed in [1], ip_tunnel_newlink can do it
properly.

Now Eric noticed that it could cover the mtu value set in do_setlink
when creating a ip_gre dev. It makes IFLA_MTU param not take effect.

So this patch is to remove them to make IFLA_MTU work, as in other
ipv4 tunnels.

  [1]: https://patchwork.ozlabs.org/patch/823504/

Fixes: c5441932 ("GRE: Refactor GRE tunneling code.")
Reported-by: NEric Garver <e@erig.me>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -970,9 +970,6 @@ static void __gre_tunnel_init(struct net_device *dev)
 
 	t_hlen = tunnel->hlen + sizeof(struct iphdr);
 
-	dev->needed_headroom	= LL_MAX_HEADER + t_hlen + 4;
-	dev->mtu		= ETH_DATA_LEN - t_hlen - 4;
-
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
 
@@ -1290,8 +1287,6 @@ static int erspan_tunnel_init(struct net_device *dev)
 		       erspan_hdr_len(tunnel->erspan_ver);
 	t_hlen = tunnel->hlen + sizeof(struct iphdr);
 
-	dev->needed_headroom = LL_MAX_HEADER + t_hlen + 4;
-	dev->mtu = ETH_DATA_LEN - t_hlen - 4;
 	dev->features		|= GRE_FEATURES;
 	dev->hw_features	|= GRE_FEATURES;
 	dev->priv_flags		|= IFF_LIVE_ADDR_CHANGE;