[NETNS]: Introduce a netns_core structure.

There's already some stuff on the struct net, that should better
be folded into netns_core structure. I'm making the per-proto inuse 
counter be per-net also, which is also a candidate for this, so 
introduce this structure and populate it a bit.

Signed-off-by: Pavel Emelyanov <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
4 files changed