gre: do not keep the GRE header around in collect medata mode

For ipgre interface in collect metadata mode, it doesn't make sense for the
interface to be of ARPHRD_IPGRE type. The outer header of received packets
is not needed, as all the information from it is present in metadata_dst. We
already don't set ipgre_header_ops for collect metadata interfaces, which is
the only consumer of mac_header pointing to the outer IP header.

Just set the interface type to ARPHRD_NONE in collect metadata mode for
ipgre (not gretap, that still correctly stays ARPHRD_ETHER) and reset
mac_header.

Fixes: a64b04d86d14 ("gre: do not assign header_ops in collect metadata mode")
Fixes: 2e15ea390e6f4 ("ip_gre: Add support to collect tunnel metadata.")
Signed-off-by: Jiri Benc <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/ip_gre.c

···
 				  iph->saddr, iph->daddr, tpi->key);
 
 	if (tunnel) {
-		skb_pop_mac_header(skb);
+		if (tunnel->dev->type != ARPHRD_NONE)
+			skb_pop_mac_header(skb);
+		else
+			skb_reset_mac_header(skb);
 		if (tunnel->collect_md) {
 			__be16 flags;
 			__be64 tun_id;
···
 		struct ip_tunnel *t = netdev_priv(dev);
 
 		t->collect_md = true;
+		if (dev->type == ARPHRD_IPGRE)
+			dev->type = ARPHRD_NONE;
 	}
 }