Fix/zebra netns del

[leonshaw]

Fix two issues during netns deletion:

1. Free global netlink message buffer only when zebra is shutting down (not when netns is deleted).
2. Clean up queued dataplane update contexts which have references to the netns.

[mjstapp]

Can you say a bit about what the "issues" are: is there ... memory that's not freed? or ... something that's freed too soon, while it might still be used during shutdown?

[leonshaw]

@mjstapp If netns VRF backend is enabled, kernel_terminate() is called when a netns (VRF) is deleted: https://github.com/FRRouting/frr/blob/741d1d009042222c59b71db27ead08387cbe8cb0/zebra/zebra_ns.c#L144-L160 So don't free the global nl_batch_tx_buf here, and move that into kernel_router_terminate() instead.

For the second issue, if don't remove queued dataplane contexts, which has reference to netlink socket of the netns being deleted, there's a race between main (removal of netlink sock) and dataplane (handling of pending contexts) thread.

[github-actions[bot]]

This PR is stale because it has been open 180 days with no activity. Comment or remove the autoclose label in order to avoid having this PR closed.

[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[leonshaw]

Remove the first commit. An identical change is merged in #18309.