[IPV6]: Eliminate difference in actions of sysctl and proc handler for conf.all.forwarding

The only difference in this case is that updating all.forwarding
causes the update in default.forwarding when done via proc, but
not via the system call.

Besides, this consolidates a good portion of code.

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