[INET]: Merge sys.net.ipv4.ip_forward and sys.net.ipv4.conf.all.forwarding

AFAIS these two entries should do the same thing - change the
forwarding state on ipv4_devconf and on all the devices.

I propose to merge the handlers together using ctl paths.

The inet_forward_change() is static after this and I move
it higher to be closer to other "propagation" helpers and
to avoid diff making patches based on { and } matching :)
i.e. - make them easier to read.

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