qcow2: fix offset overflow in qcow2_alloc_clusters_at()

When cluster size is big enough it can lead to an offset overflow
in qcow2_alloc_clusters_at(). This patch fixes it.

The allocation is stopped each time at L2 table boundary
(see handle_alloc()), so the possible maximum bytes could be

  2^(cluster_bits - 3 + cluster_bits)

cluster_bits - 3 is used to compute the number of entry by L2
and the additional cluster_bits is to take into account each
clusters referenced by the L2 entries.

so int is safe for cluster_bits<=17, unsafe otherwise.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Benoit Canet <benoit@irqsave.net>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-refcount.c

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index c974abe..8712d8b 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -676,7 +676,13 @@
     BDRVQcowState *s = bs->opaque;
     uint64_t cluster_index;
     uint64_t old_free_cluster_index;
-    int i, refcount, ret;
+    uint64_t i;
+    int refcount, ret;
+
+    assert(nb_clusters >= 0);
+    if (nb_clusters == 0) {
+        return 0;
+    }
 
     /* Check how many clusters there are free */
     cluster_index = offset >> s->cluster_bits;