ip_gre: Refactor collect metatdata mode tunnel xmit to ip_md_tunnel_xmit

Refactor collect metatdata mode tunnel xmit to the generic xmit function
ip_md_tunnel_xmit. It makes codes more generic and support more feture
such as pmtu_update through ip_md_tunnel_xmit

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>