lwtunnel: only keep the available bits when setting vxlan md->gbp

As we can see from vxlan_build/parse_gbp_hdr(), when processing metadata
on vxlan rx/tx path, only dont_learn/policy_applied/policy_id fields can
be set to or parse from the packet for vxlan gbp option.

So do the mask when set it in lwtunnel, as it does in act_tunnel_key and
cls_flower.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -554,6 +554,7 @@ static int ip_tun_parse_opts_vxlan(struct nlattr *attr,
 
 		attr = tb[LWTUNNEL_IP_OPT_VXLAN_GBP];
 		md->gbp = nla_get_u32(attr);
+		md->gbp &= VXLAN_GBP_MASK;
 		info->key.tun_flags |= TUNNEL_VXLAN_OPT;
 	}