bio-integrity: mark kintegrityd_wq highpri and CPU intensive

Work items processed by kintegrityd_wq won't block much, may burn a
lot of CPU cycles and affect IO latency.  Use alloc_workqueue() to
mark it highpri and CPU intensive with max concurrency of 1.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>