UBI: introduce volume refcounting

Add ref_count field to UBI volumes and remove weired "vol->removed"
field. This way things are better understandable and we do not have
to do whold show_attr operation under spinlock.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/kapi.c b/drivers/mtd/ubi/kapi.c
index 9c28376..780c273 100644
--- a/drivers/mtd/ubi/kapi.c
+++ b/drivers/mtd/ubi/kapi.c
@@ -152,6 +152,7 @@
 		break;
 	}
 	get_device(&vol->dev);
+	vol->ref_count += 1;
 	spin_unlock(&ubi->volumes_lock);
 
 	desc->vol = vol;
@@ -261,10 +262,11 @@
 	case UBI_EXCLUSIVE:
 		vol->exclusive = 0;
 	}
+	vol->ref_count -= 1;
 	spin_unlock(&vol->ubi->volumes_lock);
 
-	kfree(desc);
 	put_device(&vol->dev);
+	kfree(desc);
 	module_put(THIS_MODULE);
 }
 EXPORT_SYMBOL_GPL(ubi_close_volume);