drbd: Enable QUEUE_FLAG_DISCARD only if the peer can recieve P_TRIM

Allow the user of REQ_DISCARD.

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