Merge branch 'ip_tunnel-next'

wenxu says:

====================
ip_tunnel: Refactor ip_gre collect metadata xmit to ip_md_tunnel_xmit

This patchset add tunnel_dst_cache and tnl_update_pmtu feature for
ip_md_tunnel_xmit also bugfix. Then Refactor collect metatdata mode
tunnel xmit to ip_md_tunnel_xmit
====================

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