dm crypt: use WQ_HIGHPRI for the IO and crypt workqueues

Running dm-crypt with workqueues at the standard priority results in IO
competing for CPU time with standard user apps, which can lead to
pipeline bubbles and seriously degraded performance.  Move to using
WQ_HIGHPRI workqueues to protect against that.

Signed-off-by: Tim Murray <timmurray@google.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>