gre: set inner_protocol on xmit

Ensure that the inner_protocol is set on transmit so that GSO segmentation,
which relies on that field, works correctly.

This is achieved by setting the inner_protocol in gre_build_header rather
than each caller of that function. It ensures that the inner_protocol is
set when gre_fb_xmit() is used to transmit GRE which was not previously the
case.

I have observed this is not the case when OvS transmits GRE using
lwtunnel metadata (which it always does).

Fixes: 38720352412a ("gre: Use inner_proto to obtain inner header protocol")
Cc: Pravin Shelar <pshelar@ovn.org>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/gre.h

@@ -104,6 +104,7 @@ static inline void gre_build_header(struct sk_buff *skb, int hdr_len,
 
 	skb_push(skb, hdr_len);
 
+	skb_set_inner_protocol(skb, proto);
 	skb_reset_transport_header(skb);
 	greh = (struct gre_base_hdr *)skb->data;
 	greh->flags = gre_tnl_flags_to_gre_flags(flags);

net/ipv4/ip_gre.c

@@ -370,7 +370,6 @@ static void __gre_xmit(struct sk_buff *skb, struct net_device *dev,
 			 tunnel->parms.o_flags, proto, tunnel->parms.o_key,
 			 htonl(tunnel->o_seqno));
 
-	skb_set_inner_protocol(skb, proto);
 	ip_tunnel_xmit(skb, dev, tnl_params, tnl_params->protocol);
 }
 

net/ipv6/ip6_gre.c

@@ -519,8 +519,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 	gre_build_header(skb, tunnel->tun_hlen, tunnel->parms.o_flags,
 			 protocol, tunnel->parms.o_key, htonl(tunnel->o_seqno));
 
-	skb_set_inner_protocol(skb, protocol);
-
 	return ip6_tnl_xmit(skb, dev, dsfield, fl6, encap_limit, pmtu,
 			    NEXTHDR_GRE);
 }