platform: improve ipv6 addresses synchronization
nm_platform_ip6_address_sync() must take care not only of adding missing addresses and removing unknown addresses, but also of the order in which they are added. The order is important because it determines which address is preferred by kernel. Since we can only add addresses at the top of the list, in order to change the position of an address we must first remove it and then re-add it in the right position.
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