netdevsim: Restore per-network namespace accounting for fib entries

Prior to the commit in the fixes tag, the resource controller in netdevsim
tracked fib entries and rules per network namespace. Restore that behavior.

Fixes: 5fc49422 ("netdevsim: create devlink instance per netdevsim instance")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>