um: ubd: Fix for processes stuck in D state forever

Starting with Linux 3.12 processes get stuck in D state forever in
UserModeLinux under sync heavy workloads. This bug was introduced by
commit 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport).
Fix bug by adding a check if FLUSH request was successfully submitted to
the I/O thread and keeping the FLUSH request on the request queue on
submission failures.

Fixes: 805f11a0d5 (um: ubd: Add REQ_FLUSH suppport)
Signed-off-by: Thorsten Knabe <linux@thorsten-knabe.de>
Cc: stable@kernel.org # >= 3.12
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/ubd_kern.c

@@ -1277,7 +1277,7 @@ static void do_ubd_request(struct request_queue *q)
 
 	while(1){
 		struct ubd *dev = q->queuedata;
-		if(dev->end_sg == 0){
+		if(dev->request == NULL){
 			struct request *req = blk_fetch_request(q);
 			if(req == NULL)
 				return;
@@ -1299,7 +1299,8 @@ static void do_ubd_request(struct request_queue *q)
 				return;
 			}
 			prepare_flush_request(req, io_req);
-			submit_request(io_req, dev);
+			if (submit_request(io_req, dev) == false)
+				return;
 		}
 
 		while(dev->start_sg < dev->end_sg){