net: fib_notifier: Add temporary events to the FIB notification chain

Subsequent patches are going to simplify the IPv4 route offload API,
which will only use two events - replace and delete.

Introduce a temporary version of these two events in order to make the
conversion easier to review.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>