qcow2: Fix error return code in qcow2_alloc_cluster_link_l2

Fix qcow2_alloc_cluster_link_l2 to return the real error code like it does in
all other error cases.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -687,8 +687,8 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
                     (i << s->cluster_bits)) | QCOW_OFLAG_COPIED);
      }
 
-    if (write_l2_entries(s, l2_table, l2_offset, l2_index, m->nb_clusters) < 0) {
-        ret = -1;
+    ret = write_l2_entries(s, l2_table, l2_offset, l2_index, m->nb_clusters);
+    if (ret < 0) {
         goto err;
     }