qcow2: Free allocated L2 cluster on error

If an error occurs in l2_allocate, the allocated (but unused) L2 cluster
should be freed.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2-cluster.c

@@ -273,6 +273,10 @@ fail:
         qcow2_cache_put(bs, s->l2_table_cache, (void**) table);
     }
     s->l1_table[l1_index] = old_l2_offset;
+    if (l2_offset > 0) {
+        qcow2_free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t),
+                            QCOW2_DISCARD_ALWAYS);
+    }
     return ret;
 }