ip6: fix IPv6 address calculation error using "ip route add" CLI

Using VPP CLI "ip route add" to add static IPv6 entries outputs wrong
results. Fix this error by correctly calculating IPv6 addresses with
different increased ranges and grouping ip4/ip6 prefix calculation
functionality into two functions.

Type: fix

Signed-off-by: Jieqiang Wang <jieqiang.wang@arm.com>
Reviewed-by: Lijian Zhang <lijian.zhang@arm.com>
Reviewed-by: Tianyu Li <tianyu.li@arm.com>
Change-Id: If954876301ca2095f9331799a086f75db936f246