qcow2: Switch L1 table in a single sequence

Switching the L1 table in memory should be an atomic operation, as far
as possible. Calling qcow2_free_clusters on the old L1 table on disk is
not a good idea when the old L1 table is no longer valid and the address
to the new one hasn't yet been written into the corresponding
BDRVQcowState field. To be more specific, this can lead to segfaults due
to qcow2_check_metadata_overlap trying to access the L1 table during the
free operation.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2-cluster.c

@@ -35,6 +35,7 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
     BDRVQcowState *s = bs->opaque;
     int new_l1_size2, ret, i;
     uint64_t *new_l1_table;
+    int64_t old_l1_table_offset, old_l1_size;
     int64_t new_l1_table_offset, new_l1_size;
     uint8_t data[12];
 
@@ -106,11 +107,13 @@ int qcow2_grow_l1_table(BlockDriverState *bs, uint64_t min_size,
         goto fail;
     }
     g_free(s->l1_table);
-    qcow2_free_clusters(bs, s->l1_table_offset, s->l1_size * sizeof(uint64_t),
-                        QCOW2_DISCARD_OTHER);
+    old_l1_table_offset = s->l1_table_offset;
     s->l1_table_offset = new_l1_table_offset;
     s->l1_table = new_l1_table;
+    old_l1_size = s->l1_size;
     s->l1_size = new_l1_size;
+    qcow2_free_clusters(bs, old_l1_table_offset, old_l1_size * sizeof(uint64_t),
+                        QCOW2_DISCARD_OTHER);
     return 0;
  fail:
     g_free(new_l1_table);