xfrm: clone missing x->lastused in xfrm_do_migrate

x->lastused was not cloned in xfrm_do_migrate. Add it to clone during
migrate.

Fixes: 80c9abaa ("[XFRM]: Extension for dynamic update of endpoint address(es)")
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>