Fix extrinsics count logging in frame-system (#4461)
The storage item ExtrinsicIndex is already taken before the `finalize()` in `note_finished_extrinsics()`, rendering it's always 0 in the log. This commit fixes it by using the proper API for extrinsics count. --------- Co-authored-by: Bastian Köcher <[email protected]> Co-authored-by: Oliver Tale-Yazdi <[email protected]>