net: fib_notifier: move fib_notifier_ops from struct net into per-net struct

No need for fib_notifier_ops to be in struct net. It is used only by
fib_notifier as a private data. Use net_generic to introduce per-net
fib_notifier struct and move fib_notifier_ops there.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>