drbd: new disk-option disable-write-same

Some backend devices claim to support write-same,
but would fail actual write-same requests.

Allow to set (or toggle) whether or not DRBD tries to support write-same.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>