Add checksum to verify_header

Add checksum to verify_header

Currently we don't know if the verify_header itself is
valid when pulled off a disk. While it will fail verify
if it is, fio might then confuse the 'expected' and
'received' data since it re-generates the corrupted
crc with the seed stored. But if the seed is corrupt,
then we fail.

So checksum the verify header, so that we know for a fact
whether the header is valid or not.

Signed-off-by: Jens Axboe <axboe@kernel.dk>