drbd: fix check for too large lower level device

To check wether we are truncating a very large device due to limited
meta data space, we need to check the ll_dev size.

Also improve the printk to suggest "flexible" or "internal".

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>