fix iptables-restore failing if icmp-type value > 255

fix iptables-restore failing if icmp-type value > 255

This has to be done in both icmp and icmpv6 cases. Currently if
'ipv6-icmp' is set via the GUI ('icmpv6' is not available there) there
is no icmp-type handling. As this is meant to fix the iptables-restore
failure if an icmp-type > 255 is specified, no ipv6-icmp handling is
introduced.

These error messages are not logged as warnings are ignored. To get
these messages you have to run pve-firewall compile and look at the
output.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>