`addr_adv` plugin: MPTCP endpoints are not removed when the attached IP is removed

[matttbe]

Describe the bug (See issue #281 opened by @majek for more details)

With the in-kernel PM and the addr_adv plugin, it looks like mptcpd doesn't remove endpoints when the attached IP is removed

To Reproduce Steps to reproduce the behavior:

1. Use in-kernel PM and mptcpd with addr_adv plugin
2. Plug a new interface and wait for the IP to be assigned → ip mptcp endpoint show shows a new endpoint
3. Unplug it → ip mptcp endpoint show is still showing the endpoint

Expected behavior The endpoint should be removed

Desktop (please complete the following information):

• Ubuntu Jammy
• Linux Kernel: v6.4
• mptcpd compiled from Git

Additional context see #281 I'm not the one who discovered the bug, @majek did.