[IPV4]: Cleanup sysctl manipulations in devinet.c

This includes:

 * moving neigh_sysctl_(un)register calls inside
   devinet_sysctl_(un)register ones, as they are always
   called in pairs;
 * making __devinet_sysctl_unregister() to unregister
   the ipv4_devconf struct, while original devinet_sysctl_unregister()
   works with the in_device to handle both - devconf and
   neigh sysctls;
 * make stubs for CONFIG_SYSCTL=n case to get rid of
   in-code ifdefs.

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