Use correct I/O engine name "cpuio" instead of "cpu"

Use correct I/O engine name "cpuio" instead of "cpu"

"cpuio" is the name of the engine/option, though the filename is cpu.c.

 # grep "ioengine =" engines/cpu.c -A1
 static struct ioengine_ops ioengine = {
 	.name           = "cpuio",

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>