Fix extrinsics count logging in frame-system

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.

substrate/frame/system/src/lib.rs

@@ -1780,7 +1780,7 @@ impl<T: Config> Pallet<T> {
 			"[{:?}] {} extrinsics, length: {} (normal {}%, op: {}%, mandatory {}%) / normal weight:\
 			 {} ({}%) op weight {} ({}%) / mandatory weight {} ({}%)",
 			Self::block_number(),
-			Self::extrinsic_index().unwrap_or_default(),
+			Self::extrinsic_count(),
 			Self::all_extrinsics_len(),
 			sp_runtime::Percent::from_rational(
 				Self::all_extrinsics_len(),