Btrfs: Clear the device->running_pending flag before bailing on congestion

Btrfs maintains a queue of async bio submissions so the checksumming
threads don't have to wait on get_request_wait.  In order to avoid
extra wakeups, this code has a running_pending flag that is used
to tell new submissions they don't need to wake the thread.

When the threads notice congestion on a single device, they
may decide to requeue the job and move on to other devices.  This
makes sure the running_pending flag is cleared before the
job is requeued.

It should help avoid IO stalls by making sure the task is woken up
when new submissions come in.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -220,6 +220,7 @@ loop:
 				tail->bi_next = old_head;
 			else
 				device->pending_bio_tail = tail;
+			device->running_pending = 0;
 
 			spin_unlock(&device->io_lock);
 			btrfs_requeue_work(&device->work);