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>