add default link local route to fix ndp inside vrf issue

[baorliu]

What I did
Add default IPv6 link local route to fix ipv6 ndp issue inside VRF

Why I did it
When a VRF is created, all the assigned IP addresses are removed and a new link local address is regenerated in Linux kernel but SWSS does not add a route for link local fe80::/10.
Linux sends NS packet with link local addresses and peer reply NA packet also uses link local address caused the NA packet be dropped and the ndp cannot be updated.

How I verified it
Built test image with the fix and verified it in a SONiC router. The system can process replied NA packet with link local dst IP address to that VRF.

2025 Nov  5 05:59:13.630349 sonic NOTICE swss#vrfmgrd: :- doTask: Created vrf netdev Vrf01
2025 Nov  5 05:59:13.630188 sonic INFO (udev-worker)[30654]: Network interface NamePolicy= disabled on kernel command line.
2025 Nov  5 05:59:14.244172 sonic NOTICE swss#orchagent: :- addLinkLocalRouteToMe: Created link local ipv6 route  fe80::/10 to cpu
2025 Nov  5 05:59:14.244295 sonic NOTICE swss#orchagent: :- addOperation: Created link local ipv6 route fe80::/10 to cpu for VRF 'Vrf01'
2025 Nov  5 05:59:14.244359 sonic NOTICE swss#orchagent: :- addOperation: VRF 'Vrf01' was added

06:00:09.796519 Ethernet16 Out IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32) fe80::c0:a2ff:fe61:9d2 > ff02::1:ff00:2: [icmp6 sum ok] ICMP6, neighbor solicitation, length 32, who has 2001:db8:2:4::2
	  source link-address option (1), length 8 (1): 02:c0:a2:61:09:d2
	    0x0000:  02c0 a261 09d2
06:00:09.796590 Ethernet24 Out IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32) fe80::c0:a2ff:fe61:9d2 > ff02::1:ff00:5: [icmp6 sum ok] ICMP6, neighbor solicitation, length 32, who has 2001:db8:4:5::5
	  source link-address option (1), length 8 (1): 02:c0:a2:61:09:d2
	    0x0000:  02c0 a261 09d2
06:00:09.949298 Ethernet16 In  IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32) 2001:db8:2:4::2 > fe80::c0:a2ff:fe61:9d2: [icmp6 sum ok] ICMP6, neighbor advertisement, length 32, tgt is 2001:db8:2:4::2, Flags [router, solicited, override]
	  destination link-address option (2), length 8 (1): 02:e1:2b:f0:9d:2c
	    0x0000:  02e1 2bf0 9d2c
06:00:09.950745 Ethernet24 In  IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32) 2001:db8:4:5::5 > fe80::c0:a2ff:fe61:9d2: [icmp6 sum ok] ICMP6, neighbor advertisement, length 32, tgt is 2001:db8:4:5::5, Flags [router, solicited, override]
	  destination link-address option (2), length 8 (1): 02:0a:d6:dc:69:74
	    0x0000:  020a d6dc 6974

Details if related

[mssonicbld]

/azp run

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[mssonicbld]

/azp run

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