[XFRM]: Fix MTU calculation for non-ESP SAs

My IPsec MTU optimization patch introduced a regression in MTU calculation
for non-ESP SAs, the SA's header_len needs to be subtracted from the MTU if
the transform doesn't provide a ->get_mtu() function.
Reported-and-tested-by: Marco Berizzi <pupilla@hotmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_state.c

@@ -1729,7 +1729,7 @@ int xfrm_state_mtu(struct xfrm_state *x, int mtu)
 	    x->type && x->type->get_mtu)
 		res = x->type->get_mtu(x, mtu);
 	else
-		res = mtu;
+		res = mtu - x->props.header_len;
 	spin_unlock_bh(&x->lock);
 	return res;
 }