Support > 4 byte verify write patterns

This is the patch I have to write/read/verify hex patterns of size > 4
bytes.  I have tested these on hard drives and did not see any
performance change. I am assuming the performance drop if any might show
up in the read phase for probably solid state drives and maybe it can be
worked around using the asynchronous verify flag.

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