minor split optimization

let's fill the initial stats directly into target fingerprint

lib/compress/zstd_preSplit.h

@@ -22,6 +22,10 @@ extern "C" {
 /* note:
  * @workspace must be aligned on 8-bytes boundaries
  * @wkspSize must be at least >= ZSTD_SLIPBLOCK_WORKSPACESIZE
+ * note2:
+ * for the time being, this function only accepts full 128 KB blocks,
+ * therefore @blockSizeMax must be == 128 KB.
+ * This could be extended to smaller sizes in the future.
  */
 size_t ZSTD_splitBlock_4k(const void* src, size_t srcSize, size_t blockSizeMax, void* workspace, size_t wkspSize);