Cleanup after MDEV-12026

buf_flush_init_for_writing(): If innodb_checksum_algorithm=full_crc32,
return even earlier.

The logic to reset garbage FIL_PAGE_TYPE fields applies to
data pages in old data files that were last written before
commit 3926673c was introduced.

When innodb_checksum_algorithm=full_crc32 is in effect, it only
applies to newly created data files. A file that was created in
full_crc32 format will keep that format. Other files will keep
some other format than full_crc32. Hence, the garbage-resetting
logic does not apply full_crc32 files.

storage/innobase/buf/buf0flu.cc

@@ -838,6 +838,7 @@ buf_flush_init_for_writing(
 	if (use_full_checksum) {
 		mach_write_to_4(page + srv_page_size - FIL_PAGE_FCRC32_END_LSN,
 				static_cast<uint32_t>(newest_lsn));
+		return buf_flush_assign_full_crc32_checksum(page);
 	} else {
 		mach_write_to_8(page + srv_page_size - FIL_PAGE_END_LSN_OLD_CHKSUM,
 				newest_lsn);
@@ -907,10 +908,6 @@ buf_flush_init_for_writing(
 
 	uint32_t checksum = BUF_NO_CHECKSUM_MAGIC;
 
-	if (use_full_checksum) {
-		return buf_flush_assign_full_crc32_checksum(page);
-	}
-
 	switch (srv_checksum_algorithm_t(srv_checksum_algorithm)) {
 	case SRV_CHECKSUM_ALGORITHM_INNODB:
 	case SRV_CHECKSUM_ALGORITHM_STRICT_INNODB: