devlink->dev is assumed to be always valid as long as any
outstanding reference to the devlink instance exists.

In prep for weakening of the references take the instance lock.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>