DHCPv6: Update the snapend for nested DHCPv6 packets

DHCPv6: Update the snapend for nested DHCPv6 packets

Update the snapend to the end of the option before calling recursively
dhcp6_print() for the nested packet.
Other options may be present after the nested DHCPv6 packet.
This prevents that, in dhcp6_print(), for the nested DHCPv6 packet,
the remaining length < remaining caplen.