qcow2: Clear L2 table cache after write error

If the L2 table was already updated in cache, but writing it to disk has
failed, we must not continue using the changed version in the cache to stay
consistent with what's on the disk.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -696,6 +696,7 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
 
     ret = write_l2_entries(bs, l2_table, l2_offset, l2_index, m->nb_clusters);
     if (ret < 0) {
+        qcow2_l2_cache_reset(bs);
         goto err;
     }