[NET] sysctl: prepare core tables to point to netns variables

Some of ctl variables are going to be on the struct
net. Here's the way to adjust the ->data pointer on the
ctl_table-s to point on the right variable.

Since some pointers still point on the global variables,
I keep turning the write bits off on such tables.

This looks to become a common procedure for net sysctls,
so later parts of this code may migrate to some more
generic place.

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