qcow2: Fix corruption on write_zeroes with MAY_UNMAP

Commit 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()")
introduced a subtle change to code in zero_in_l2_slice:

It swapped the order of

1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
2. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);
3. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);

To

1. qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
2. qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
3. set_l2_entry(s, l2_slice, l2_index + i, QCOW_OFLAG_ZERO);

It seems harmless, however the call to qcow2_free_any_clusters can
trigger a cache flush which can mark the L2 table as clean, and
assuming that this was the last write to it, a stale version of it
will remain on the disk.

Now we have a valid L2 entry pointing to a freed cluster. Oops.

Fixes: 205fa50750 ("qcow2: Add subcluster support to zero_in_l2_slice()")
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
[ kwolf: Fixed to restore the correct original order from before
  205fa50750; added comments like in discard_in_l2_slice(). ]
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20201124092815.39056-1-kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -2010,14 +2010,17 @@ static int zero_in_l2_slice(BlockDriverState *bs, uint64_t offset,
             continue;
         }
 
+        /* First update L2 entries */
         qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_slice);
-        if (unmap) {
-            qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST);
-        }
         set_l2_entry(s, l2_slice, l2_index + i, new_l2_entry);
         if (has_subclusters(s)) {
             set_l2_bitmap(s, l2_slice, l2_index + i, new_l2_bitmap);
         }
+
+        /* Then decrease the refcount */
+        if (unmap) {
+            qcow2_free_any_cluster(bs, old_l2_entry, QCOW2_DISCARD_REQUEST);
+        }
     }
 
     qcow2_cache_put(s->l2_table_cache, (void **) &l2_slice);