MDEV-21024: Optimize writing BTR_EXTERN_LEN

btr_store_big_rec_extern_fields(): Remove the redundant initialization
of the most significant 32 bits of BTR_EXTERN_LEN. InnoDB never supported
BLOBs that are longer than 4GiB. In fact, dtuple_convert_big_rec()
would write emit an error message if a clustered index record tuple would
exceed 1,000,000,000 bytes in length.

The BTR_EXTERN_LEN in the BLOB pointers in clustered index leaf page
records is zero-initialized at least since
commit 41bb3537

storage/innobase/btr/btr0cur.cc

@@ -7231,8 +7231,8 @@ btr_store_big_rec_extern_fields(
 
 				extern_len -= store_len;
 
-				mlog_write_ulint(field_ref + BTR_EXTERN_LEN, 0,
-						 MLOG_4BYTES, &mtr);
+				ut_ad(!mach_read_from_4(BTR_EXTERN_LEN
+							+ field_ref));
 				mlog_write_ulint(field_ref
 						 + BTR_EXTERN_LEN + 4,
 						 big_rec_vec->fields[i].len