devlink: take device reference for devlink object

In preparation to allow to access device pointer without devlink
instance lock held, make sure the device pointer is usable until
devlink_release() is called.

Fixes: c137743b ("devlink: introduce object and nested devlink relationship infra")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>