qcow2: fix possible corruption when reading multiple clusters

if multiple sectors spanning multiple clusters are read the
function count_contiguous_clusters should ensure that the
cluster type should not change between the clusters.

Especially the for-loop should break when we have one
or more normal clusters followed by a compressed cluster.

Unfortunately the wrong macro was used in the mask to
compare the flags.

This was discovered while debugging a data corruption
issue when converting a compressed qcow2 image to raw.
qemu-img reads 2MB chunks which span multiple clusters.

CC: qemu-stable@nongnu.org
Signed-off-by: NPeter Lieven <pl@kamp.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
(cherry picked from commit 78a52ad5)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

block/qcow2-cluster.c

@@ -266,7 +266,7 @@ static int count_contiguous_clusters(uint64_t nb_clusters, int cluster_size,
         uint64_t *l2_table, uint64_t start, uint64_t stop_flags)
 {
     int i;
-    uint64_t mask = stop_flags | L2E_OFFSET_MASK | QCOW2_CLUSTER_COMPRESSED;
+    uint64_t mask = stop_flags | L2E_OFFSET_MASK | QCOW_OFLAG_COMPRESSED;
     uint64_t first_entry = be64_to_cpu(l2_table[0]);
     uint64_t offset = first_entry & mask;