[Neighbor Advertiser] IPv6 link-local not advertised

[prsunny]

Description

IPv6 link-local addresses are not advertised by the script during warmboot as the current approach is fetching the data from config_db. Need to get the link-local for the Vlan interface from kernel and add to mapping. This is required for NS/ND to work during warmboot.

Steps to reproduce the issue:

1. Run neighbor advertiser script
2. Check the advertised mapping.

Describe the results you received:

Describe the results you expected:

Additional information you deem important (e.g. issue happens only occasionally):

**Output of `show version`:**

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**Attach debug file `sudo generate_dump`:**

```
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