When implementing IPv6-related logic, we found the `net_ipv6_pref2mask`
and `net_ipv6addr_copy` are using different argument order:
```
net_ipv6addr_copy(ifaddr->addr, addr);
net_ipv6_pref2mask(preflen, ifaddr->mask);
```
Change the order to:
```
net_ipv6addr_copy(ifaddr->addr, addr);
net_ipv6_pref2mask(ifaddr->mask, preflen);
```
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>