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

Subsequent patches are going to simplify the IPv6 route offload API,
which will only use three events - replace, delete and append.

Introduce a temporary version of replace and delete in order to make the
conversion easier to review. Note that append does not need a temporary
version, as it is currently not used.

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