isisd: Request SRv6 locator after zebra connection

isisd: Request SRv6 locator after zebra connection

When SRv6 is enabled and an SRv6 locator is specified in the IS-IS
configuration, IS-IS may attempt to request SRv6 locator information from
zebra before the connection is fully established. If this occurs, the
request fails with the following error:

```
2025/02/14 21:41:20 ISIS: [HR66R-TWQYD][EC 100663302] srv6_manager_get_locator: invalid zclient socket
````

As a result, IS-IS is unable to obtain the locator information,
preventing SRv6 from working.

This commit fixes the issue by ensuring IS-IS requests SRv6 locator
information once the connection with zebra is successfully established.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>