drbd: fix spurious meta data IO "error"

When detaching, even cleanly detaching due to administrator request,
we always go through D_FAILED before we become D_DISKLESS.

Don't let that state change race with an in-flight meta data IO,
or that one might think it actually experienced an IO error.

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