devlink: expose instance locking and add locked port registering

It should be familiar and beneficial to expose devlink instance
lock to the drivers. This way drivers can block devlink from
calling them during critical sections without breakneck locking.

Add port helpers, port splitting callbacks will be the first
target.

Use 'devl_' prefix for "explicitly locked" API. Initial RFC used
'__devlink' but that's too much typing.

devl_lock_is_held() is not defined without lockdep, which is
the same behavior as lockdep_is_held() itself.

Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/devlink/index.rst

@@ -4,6 +4,22 @@ Linux Devlink Documentation
 devlink is an API to expose device information and resources not directly
 related to any device class, such as chip-wide/switch-ASIC-wide configuration.
 
+Locking
+-------
+
+Driver facing APIs are currently transitioning to allow more explicit
+locking. Drivers can use the existing ``devlink_*`` set of APIs, or
+new APIs prefixed by ``devl_*``. The older APIs handle all the locking
+in devlink core, but don't allow registration of most sub-objects once
+the main devlink object is itself registered. The newer ``devl_*`` APIs assume
+the devlink instance lock is already held. Drivers can take the instance
+lock by calling ``devl_lock()``. It is also held in most of the callbacks.
+Eventually all callbacks will be invoked under the devlink instance lock,
+refer to the use of the ``DEVLINK_NL_FLAG_NO_LOCK`` flag in devlink core
+to find out which callbacks are not converted, yet.
+
+Drivers are encouraged to use the devlink instance lock for their own needs.
+
 Interface documentation
 -----------------------