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.