gre: strip gre-tso offload flags

if the gro enable, ipgre receive a gre-tso package. After pop
the gre-tunnel the encapsulation and GSO_ENCAP flags should be
striped. or the packet encap again and will be dropped in
ovs_iptunnel_handle_offloads

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>

datapath/linux/compat/ip_gre.c

@@ -140,6 +140,8 @@ static int ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi)
 		__be64 tun_id;
 		int err;
 
+		if (iptunnel_pull_offloads(skb))
+			return PACKET_REJECT;
 
 		skb_pop_mac_header(skb);
 		flags = tpi->flags & (TUNNEL_CSUM | TUNNEL_KEY);