ppp: propagate master firewall zone to dynamic slave interface

ppp: propagate master firewall zone to dynamic slave interface

Assign the virtual DHCPv6 interface the firewall zone of the parent interface
so fw3 knows the zone to which the virtual DHCPv6 interface belongs.
This guarantees the firewall settings are applied correctly for the virtual
DHCPv6 interface and allows to query the zone to which the virtual DHCPv6
interface belongs via the fw3 network option.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
(cherry picked from commit 21f25bc4a325b3075d6426047640e9cf6176894c)