[SOCK][NETNS]: Add the percpu prot_inuse counter in the struct net.

Such an accounting would cost us two more dereferences to get the
percpu variable from the struct net, so I make sock_prot_inuse_get
and _add calls work differently depending on CONFIG_NET_NS - without
it old optimized routines are used.

The per-cpu counter for init_net is prepared in core_initcall, so
that even af_inet, that starts as fs_initcall, will already have the
init_net prepared.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>