dm-verity: run in a WQ_HIGHPRI workqueue

Running dm-verity in a standard workqueue results in IO competing for
CPU time with standard user apps, which can lead to pipeline bubbles and
seriously degraded performance. Move to a WQ_HIGHPRI workqueue to
protect against that.

bug 25392275

Change-Id: Ic65d7bd6f04e4d77780119e926a50e71323575f0