[NET] sysctl: make the sys.net.core sysctls per-namespace

Making them per-namespace is required for the following
two reasons:

 First, some ctl values have a per-namespace meaning.
 Second, making them writable from the sub-namespace
 is an isolation hole.

So I introduce the pernet operations to create these
tables. For init_net I use the existing statically
declared tables, for sub-namespace they are duplicated
and the write bits are removed from the mode.

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

include/net/net_namespace.h

@@ -37,6 +37,9 @@ struct net {
 
 	struct sock 		*rtnl;			/* rtnetlink socket */
 
+	/* core sysctls */
+	struct ctl_table_header	*sysctl_core_hdr;
+
 	/* List of all packet sockets. */
 	rwlock_t		packet_sklist_lock;
 	struct hlist_head	packet_sklist;