ipv4: expose IPV4_DEVCONF

IP sends device configuration (see inet_fill_link_af) as an array
in the netlink information, but the indices in that array are not
exposed to userspace through any current santized header file.

It was available back in 2.6.32 (in /usr/include/linux/sysctl.h)
but was broken by:
  commit 02291680
  Author: Eric W. Biederman <ebiederm@xmission.com>
  Date:   Sun Feb 14 03:25:51 2010 +0000

    net ipv4: Decouple ipv4 interface parameters from binary sysctl numbers

Eric was solving the sysctl problem but then the indices were re-exposed
by a later addition of devconf support for IPV4

  commit 9f0f7272
  Author: Thomas Graf <tgraf@infradead.org>
  Date:   Tue Nov 16 04:32:48 2010 +0000

    ipv4: AF_INET link address family

Putting them in /usr/include/linux/ip.h seemed the logical match
for the DEVCONF_ definitions for IPV6 in /usr/include/linux/ip6.h

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>