IPv6: ignore some LL addresses by default?

[matttbe]

Describe the bug
By default, with addr_adv PM (I didn't check the other ones), all IPv6 addresses are added as endpoints.

In most cases, there is no reason to announce local addresses, e.g. fe80::/10. Maybe it would be better to ignore them by default?

There are probably more than just the IPv6 LL addresses: in v4, there are also some link locals: 169.254.0.0/16
In v6, there are the ULA: fc00::/7

https://en.wikipedia.org/wiki/Reserved_IP_addresses

Not sure about the default behaviour with private IPs

To Reproduce
On a system with IPv6 configured with mptcpd and addr_adv PM, just check the MPTCP endpoints.

Expected behavior
By default, no LL addresses.

Screenshots
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Desktop (please complete the following information):

• Linux Kernel: 5.19
• Debian/Ubuntu packages

Additional context
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