xfrm: Allow user space config of SAD mark

Add ability for netlink userspace to manipulate the SAD
and manipulate the mark, retrieve it and get events with a defined
mark.
MIGRATE may be added later.

Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>