net: devlink: allow to change namespaces during reload

All devlink instances are created in init_net and stay there for a
lifetime. Allow user to be able to move devlink instances into
namespaces during devlink reload operation. That ensures proper
re-instantiation of driver objects, including netdevices.

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>