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>