xfrm: force a dst reference in __xfrm_route_forward()

Packets going through __xfrm_route_forward() have a not refcounted dst
entry, since we enabled a noref forwarding path.

xfrm_lookup() might incorrectly release this dst entry.

It's a bit late to make invasive changes in xfrm_lookup(), so lets force
a refcount in this path.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>