[IPV4]: fib_sync_down rework.

fib_sync_down can be called with an address and with a device. In
reality it is called either with address OR with a device. The
codepath inside is completely different, so lets separate it into two
calls for these two cases.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>