drbd: Do not display bogus log lines for pdsk in case pdsk < D_UNKNOWN

This was a regression recently introduced with commit
7848ddb752c09b6dfd1ddfabb06b69b08aa8f6b9
"drbd: Correctly handle resources without volumes"

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