qcow2: Restore L1 entry on l2_allocate failure

If writing the L1 table to disk failed, we need to restore its old content in
memory to avoid inconsistencies.

Reported-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -285,6 +285,7 @@ static int l2_allocate(BlockDriverState *bs, int l1_index, uint64_t **table)
     return 0;
 
 fail:
+    s->l1_table[l1_index] = old_l2_offset;
     qcow2_l2_cache_reset(bs);
     return ret;
 }