net: devlink: introduce "unregistering" mark and use it during devlinks iteration

Add new mark called "unregistering" to be set at the beginning of
devlink_unregister() function. Check this mark during devlinks
iteration in order to prevent getting a reference of devlink which is
being currently unregistered.

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