Before commit c0f47a4a (MDEV-12026)
introduced the innodb_checksum_algorithm=full_crc32 format,
it was impossible to tell if InnoDB data files contained garbage in
the FIL_PAGE_TYPE header field (and possibly other fields).
This is because before commit 3926673c
in MySQL 5.1.48, InnoDB would write uninitialized data to some fields,
and because there was no way to tell with which InnoDB version a data
file was created.

If fil_space_t::full_crc32() holds, the data file cannot contain
uninitialized garbage or invalid FIL_PAGE_TYPE, and thus
fil_block_check_type() should not be invoked to correct anything.