bgpd: Request SRv6 locator after zebra connection

bgpd: Request SRv6 locator after zebra connection

When SRv6 is enabled and an SRv6 locator is specified in the BGP
configuration, BGP 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:

```
2025/02/06 16:37:32 BGP: [HR66R-TWQYD][EC 100663302] srv6_manager_get_locator: invalid zclient socket
````

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

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

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>
(cherry picked from commit 16640b615dfabfd8e18dd091b1d4a63dfa7bf9fe)