[PATCH] Offset verification header by a user-specified distance

[PATCH] Offset verification header by a user-specified distance

Offset verification header by user specified distance.

 - Implementation is somewhat simple and probably not ideal but it works. The
   header is just swapped with the bytes at offset after the chunk has been
   filled during populate. Everything is swapped back before verify.

 - Also fixes a bug where we were relying on a moving pointer for increment
   size in populate_verify_io_u (which was working until this patch).

 - Also cleans up a couple smalls things from the header_interval patch.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>