netdevsim: take devlink net instead of init_net

Follow-up patch is going to allow to reload devlink instance into
different network namespace, so use devlink_net() helper instead
of init_net.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>