sql: fix a small race in distIndexBackfill

This is another small data race in distIndexBackfill. This code was
operating on mu.updatedTodoSpans (without holding the lock) instead of
the copy that we created a few lines above (while holding the lock).

Release note: None

pkg/sql/backfill.go

@@ -1056,7 +1056,7 @@ func (sc *SchemaChanger) distIndexBackfill(
 		if updatedTodoSpans == nil {
 			return nil
 		}
-		nRanges, err := numRangesInSpans(ctx, sc.db, sc.distSQLPlanner, mu.updatedTodoSpans)
+		nRanges, err := numRangesInSpans(ctx, sc.db, sc.distSQLPlanner, updatedTodoSpans)
 		if err != nil {
 			return err
 		}