lib: fix false context information for SRv6 route (backport #18023)

[mergify]

The seg6local route dumped by 'show ipv6 route' makes think that the USP flavor is supported, whereas it is not the case. This information is a context information, and for End, the context information should be empty.

show ipv6 route

[..]
I>* fc00:0:4::/128 [115/0] is directly connected, sr0, seg6local End USP, weight 1, 00:49:01

Fix this by suppressing the USP information from the output.

Fixes: e496b42 ("bgpd: prefix-sid srv6 l3vpn service tlv")

---

This is an automatic backport of pull request #18023 done by Mergify.

[mergify]

Cherry-pick of 658bf02 has failed:

On branch mergify/bp/stable/10.1/pr-18023
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	both modified:   lib/srv6.c

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[cscarpitta]

Replaced by #18175