drbd: set QUEUE_FLAG_STABLE_WRITES

[ Upstream commit 28690194 ]

We want our pages not to change while they are being written.

Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>