xfrm: interface: support collect metadata mode

This commit adds support for 'collect_md' mode on xfrm interfaces.

Each net can have one collect_md device, created by providing the
IFLA_XFRM_COLLECT_METADATA flag at creation. This device cannot be
altered and has no if_id or link device attributes.

On transmit to this device, the if_id is fetched from the attached dst
metadata on the skb. If exists, the link property is also fetched from
the metadata. The dst metadata type used is METADATA_XFRM which holds
these properties.

On the receive side, xfrmi_rcv_cb() populates a dst metadata for each
packet received and attaches it to the skb. The if_id used in this case is
fetched from the xfrm state, and the link is fetched from the incoming
device. This information can later be used by upper layers such as tc,
ebpf, and ip rules.

Because the skb is scrubed in xfrmi_rcv_cb(), the attachment of the dst
metadata is postponed until after scrubing. Similarly, xfrm_input() is
adapted to avoid dropping metadata dsts by only dropping 'valid'
(skb_valid_dst(skb) == true) dsts.

Policy matching on packets arriving from collect_md xfrmi devices is
done by using the xfrm state existing in the skb's sec_path.
The xfrm_if_cb.decode_cb() interface implemented by xfrmi_decode_session()
is changed to keep the details of the if_id extraction tucked away
in xfrm_interface.c.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>