SNMP support for Huawei CE7850-32Q-EI
do not call permitted() in createIPv4Prefix

There is no way to create an IP network other than call to
createIPv4Prefix/createIPv6Prefix. Unfortunately, these functions behave
more like an ophandler, not a DB API. They called permitted() with the
given navigation information. Thus a call to them from the script_mode
fails to bind a VLAN.

This change moves the check for VLAN binding permission to the
correspongind ophandlers. The config var VLANIPV4NET_LISTSRC is checked
instead of the navigation path.