drbd: serialize admin requests for new verify run with pending bitmap io

This is an addendum to
 drbd: serialize admin requests for new resync with pending bitmap io

It avoids a race that could trigger "FIXME" assert log messages.

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