bgpd: fix do not use srv6 SID for NHT when SID is ours

bgpd: fix do not use srv6 SID for NHT when SID is ours

The resulting VPN prefix of a BGP route from a L3VPN in an srv6 setup
is not advertised to remote devices.

> r1# show bgp ipv6 vpn
> BGP table version is 2, local router ID is 1.1.1.1, vrf id 0
> Default local pref 100, local AS 65500
> Status codes:  s suppressed, d damped, h history, u unsorted, * valid, > best, = multipath,
>                i internal, r RIB-failure, S Stale, R Removed
> Nexthop codes: @NNN nexthop's vrf id, < announce-nh-self
> Origin codes:  i - IGP, e - EGP, ? - incomplete
> RPKI validation codes: V valid, I invalid, N Not found
>
>      Network          Next Hop            Metric LocPrf Weight Path
> Route Distinguisher: 1:10
>      2011:1::/64      2001:1::2@6<             0    100      0 i
>     UN=2001:1::2 EC{99:99} label=4096 sid=2001:db8:1:1:: sid_structure=[40,24,8,0] type=bgp, subtype=5

What happens is that the SID of this BGP update is used as nexthop.
Consequently, the prefix is not valid because of nexthop unreachable.
obviously the locator prefix is not reachable in that L3VRF, and the
real nexthop 2001:1::2 should be used.

> r1# show bgp vrf vrf10 nexthop  detail
> Current BGP nexthop cache:
>  2001:db8:1:1:100:: invalid, #paths 1
>   Last update: Fri Mar 14 21:18:59 2025
>   Paths:
>     2/3 2011:1::/64 RD 1:10 VRF default flags 0x4000

Fix this by considering the SID of a given BGP update, only if the SID
is not ours.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>