devlink: introduce object and nested devlink relationship infra

It is a bit tricky to maintain relationship between devlink objects and
nested devlink instances due to following aspects:

1) Locking. It is necessary to lock the devlink instance that contains
   the object first, only after that to lock the nested instance.
2) Lifetimes. Objects (e.g devlink port) may be removed before
   the nested devlink instance.
3) Notifications. If nested instance changes (e.g. gets
   registered/unregistered) the nested-in object needs to send
   appropriate notifications.

Resolve this by introducing an xarray that holds 1:1 relationships
between devlink object and related nested devlink instance.
Use that xarray index to get the object/nested devlink instance on
the other side.

Provide necessary helpers:
devlink_rel_nested_in_add/clear() to add and clear the relationship.
devlink_rel_nested_in_notify() to call the nested-in object to send
	notifications during nested instance register/unregister/netns
	change.
devlink_rel_devlink_handle_put() to be used by nested-in object fill
	function to fill the nested handle.

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