xtrabackup_56 defaults to innodb_checksum_algorithm=crc32

xtrabackup_56 uses 0 (i.e. SRV_CHECKSUM_ALGORITHM_CRC32) as the default value for srv_checksum_algorithm. This only works because InnoDB allows both 'traditional' and CRC32 checksums in this case at the cost of slower page validation (since both checksums have to be calculated to validate a page with a different algorithm value).

This will fail if innodb_checksum_algorithm is changed to strict_innodb after a restore.