zebra: Don't bind netns ID netlink socket to mcast groups

[leonshaw]

The netlink socket in zebra_ns_id_get() is only used to get netns id. There's no need to subscribe to multicast groups. In some heavy-loaded cases, unexpected notification races with normal reply message and causes failure:

zebra[110338]: [NMNEE-JJKPF][EC 4043309094] netlink recvmsg: bad sequence number 505ac8 (expected 66ea845f)

BTW, to subscribe to RTNLGRP_NSID, nl_groups should be 1 << (RTNLGRP_NSID - 1). This code has never worked as expected.

[donaldsharp]

@pguibert6WIND please take a look at this and decide if this is right or not

[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.

[leonshaw]

@pguibert6WIND @donaldsharp @riw777 What to do with this PR now? The code change is obvious indeed.