drbd: fix spurious protocol error

If we cannot satisfy a request (because our disk just broke),
we still need to drain the payload.  Or we'll get a protocol error
when interpreting the payload as DRBD packet header.

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