qcow2: Use sizeof(**refcount_table)

When implementing variable refcounts, we want to be able to easily find
all the places in qemu which are tied to a certain refcount order.
Replace sizeof(uint16_t) in the check code by sizeof(**refcount_table)
so we can later find it more easily.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-refcount.c

@@ -1647,7 +1647,7 @@ static int check_refblocks(BlockDriverState *bs, BdrvCheckResult *res,
                                                   *nb_clusters);
                         memset(&(*refcount_table)[old_nb_clusters], 0,
                                (*nb_clusters - old_nb_clusters) *
-                               sizeof(uint16_t));
+                               sizeof(**refcount_table));
                     }
                     (*refcount_table)[cluster]--;
                     inc_refcounts(bs, res, *refcount_table, *nb_clusters,