The ipv4_devconf_(all) and ipv4_devconf_dflt are currently
global, but should be per-namespace.

This set moves them on the struct net. Or, more precisely,
on the struct netns_ipv4, which in turn is on the struct net.

There are two minor things that are to be done additionally
to this set:

 1. The snmp_seq_show() needs the IPV4_DEVCONF_ALL(FORWARDING)
    value, but since this entry is still global no valid struct 
    net can be get in it, so I use the init_net's one. After 
    snmp is made per-namespace, this will be fixed easily.

 2. The rt_fill_info() needs the IPV4_DEVCONF_ALL(MC_FORWARDING),
    but the routing code is not tuned to work inside namespaces
    yet, so I use the init_net in it as well. Denis is currently
    working on ipv4 routing, so this will be prepared shortly.

Happily, all the other places of devconf-s usage can provide a
struct net pointer.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>