gre: Set inner mac header in gro complete

Set the inner mac header to point to the GRE payload when
doing GRO. This is needed if we proceed to send the packet
through GRE GSO which now uses the inner mac header instead
of inner network header to determine the length of encapsulation
headers.

Fixes: 14051f04 ("gre: Use inner mac length when computing tunnel length")
Reported-by: NWolfgang Walter <linux@stwm.de>
Signed-off-by: NTom Herbert <therbert@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/gre_offload.c

@@ -247,6 +247,9 @@ static int gre_gro_complete(struct sk_buff *skb, int nhoff)
 		err = ptype->callbacks.gro_complete(skb, nhoff + grehlen);
 
 	rcu_read_unlock();
+
+	skb_set_inner_mac_header(skb, nhoff + grehlen);
+
 	return err;
 }