ipsec: Do not reject connections in on-demand mode

ipsec: Do not reject connections in on-demand mode

When an on-demand VPN connection is not up, the packets will
traverse the firewall and be rejected by the IPSECBLOCK chain
which will cause that an ICMP error message will be sent to
the client. If that does not happen and the packet is being
silently dropped, the client will retransmit and by then
the VPN connection will hopefully be up.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>