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>
(cherry picked from commit 68dba0bf45)

block/qcow2-cluster.c

@@ -266,6 +266,7 @@ static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)
     return l2_table;
 
 fail:
+    s->l1_table[l1_index] = old_l2_offset;
     qcow2_l2_cache_reset(bs);
     return NULL;
 }