ip: xfrm: support adding xfrm metadata as lwtunnel info in routes

ip: xfrm: support adding xfrm metadata as lwtunnel info in routes

Support for xfrm metadata as lwtunnel metadata was added in kernel commit
2c2493b9da91 ("xfrm: lwtunnel: add lwtunnel support for xfrm interfaces in collect_md mode")

This commit adds the respective support in lwt routes.

Example use (consider ipsec1 as an xfrm interface in "external" mode):

ip route add 10.1.0.0/24 dev ipsec1 encap xfrm if_id 1

Or in the context of vrf, one can also specify the "link" property:

ip route add 10.1.0.0/24 dev ipsec1 encap xfrm if_id 1 link_dev eth15

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David Ahern <dsahern@kernel.org>