[NETNS]: struct net content re-work (v3)

Recently David Miller and Herbert Xu pointed out that struct net becomes
overbloated and un-maintainable. There are two solutions:
- provide a pointer to a network subsystem definition from struct net.
  This costs an additional dereferrence
- place sub-system definition into the structure itself. This will speedup
  run-time access at the cost of recompilation time

The second approach looks better for us. Other sub-systems will follow.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>