ipv4: Replace route in list before notifying

Subsequent patches will add an offload / trap indication to routes which
will signal if the route is present in hardware or not.

After programming the route to the hardware, drivers will have to ask
the IPv4 code to set the flags by passing the route's key.

In the case of route replace, the new route is notified before it is
actually inserted into the FIB alias list. This can prevent simple
drivers (e.g., netdevsim) that program the route to the hardware in the
same context it is notified in from being able to set the flag.

Solve this by first inserting the new route to the list and rollback the
operation in case the route was vetoed.

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>