netns refcounting

[Denis V. Lunev]

Hello, All!

During port of Eric's patches I have noticed an interesting thing.
The patch "net: Teach the ipv4 route cache to handle multiple network
namespaces" call hold_net for each IPv4 DST cache entry.

Though it is not possible to stop a namespace without stopping all the
devices inside. Additionally, the device can't be unregistered if there
are dst entries to it. These entries are moved to a namespace loopback
and the namespace will block until these entries will gone from a loopback.

So, I do not see a necessity to have an extra atomic on this hot path,
i.e. hold_net can re moved away for this. Are there any holes?

Regards,
	Den