qcow2: Discard preallocated zero clusters

In discard_single_l2(), we completely discard normal clusters instead of
simply turning them into preallocated zero clusters. That means we
should probably do the same with such preallocated zero clusters:
Discard them instead of keeping them allocated.
Reported-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -1511,7 +1511,8 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
                 break;
 
             case QCOW2_CLUSTER_ZERO:
-                if (!full_discard) {
+                /* Preallocated zero clusters should be discarded in any case */
+                if (!full_discard && (old_l2_entry & L2E_OFFSET_MASK) == 0) {
                     continue;
                 }
                 break;