s390/dasd: fix hanging safe offline

The safe offline processing may hang forever because it waits for I/O
which can not be started because of the offline flag that prevents new
I/O from being started.

Allow I/O to be started during safe offline processing because in this
special case we take care that the queues are empty before throwing away
the device.

Signed-off-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd.c

@@ -1965,8 +1965,12 @@ static int __dasd_device_is_unusable(struct dasd_device *device,
 {
 	int mask = ~(DASD_STOPPED_DC_WAIT | DASD_UNRESUMED_PM);
 
-	if (test_bit(DASD_FLAG_OFFLINE, &device->flags)) {
-		/* dasd is being set offline. */
+	if (test_bit(DASD_FLAG_OFFLINE, &device->flags) &&
+	    !test_bit(DASD_FLAG_SAFE_OFFLINE_RUNNING, &device->flags)) {
+		/*
+		 * dasd is being set offline
+		 * but it is no safe offline where we have to allow I/O
+		 */
 		return 1;
 	}
 	if (device->stopped) {