wg-quick: linux: use already configured addresses instead of in-memory

wg-quick: linux: use already configured addresses instead of in-memory

The ADDRESSES array might not have addresses added during PreUp. But
moreover, nft(8) and iptables(8) don't like ip addresses in the form
somev6prefix::someipv4suffix, such as fd00::1.2.3.4, while ip(8) can
handle it. So by adding these first and then asking for them back, we
always get normalized addresses suitable for nft(8) and iptables(8).

Reported-by: Silvan Nagl <mail@53c70r.de>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>