selftests: devlink_lib: add check for devlink device existence

If user passes devlink handle over DEVLINK_DEV variable, check if the
device exists.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20210527105515.790330-1-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>