qcow2: Fix order in L2 table COW

When copying L2 tables (this happens only with internal snapshots), the order
wasn't completely safe, so that after a crash you could end up with a L2 table
that has too low refcount, possibly leading to corruption in the long run.

This patch puts the operations in the right order: First allocate the new
L2 table and replace the reference, and only then decrease the refcount of the
old table.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -515,13 +515,16 @@ static int get_cluster_table(BlockDriverState *bs, uint64_t offset,
             return ret;
         }
     } else {
-        /* FIXME Order */
-        if (l2_offset)
-            qcow2_free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t));
+        /* First allocate a new L2 table (and do COW if needed) */
         ret = l2_allocate(bs, l1_index, &l2_table);
         if (ret < 0) {
             return ret;
         }
+
+        /* Then decrease the refcount of the old table */
+        if (l2_offset) {
+            qcow2_free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t));
+        }
         l2_offset = s->l1_table[l1_index] & ~QCOW_OFLAG_COPIED;
     }