BACKPORT: dm verity: enable WQ_HIGHPRI on verify_wq

WQ_HIGHPRI increases throughput and decreases disk latency when using
dm-verity. This is important in Android for camera startup speed.

The following tests were run by doing 60 seconds of random reads using
a dm-verity device backed by two ramdisks.

Without WQ_HIGHPRI
lat (usec): min=13, max=3947, avg=69.53, stdev=50.55
READ: bw=51.1MiB/s (53.6MB/s), 51.1MiB/s-51.1MiB/s (53.6MB/s-53.6MB/s)

With WQ_HIGHPRI:
lat (usec): min=13, max=7854, avg=31.15, stdev=30.42
READ: bw=116MiB/s (121MB/s), 116MiB/s-116MiB/s (121MB/s-121MB/s)

Further testing was done by measuring how long it takes to open a
camera on an Android device.

Without WQ_HIGHPRI
Total verity work queue wait times (ms):
880.960, 789.517, 898.852

With WQ_HIGHPRI:
Total verity work queue wait times (ms):
528.824, 439.191, 433.300

The average time to open the camera is reduced by 350ms (or 40-50%).

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Bug: 233247259
(cherry picked from commit afd41fff)
[nhuck: Resolved minor conflict in drivers/md/dm-verity-target.c ]
Signed-off-by: Nathan Huckleberry <nhuck@google.com>
(cherry picked from https://android-review.googlesource.com/q/commit:fd4631f8568f5bf16ac5c8b0686d40fad6e4e047)
Merged-In: I7d600c924b4a3e793b9a26c2852139683061a831

Change-Id: I94b44e6ff2384aa067948068f7bd7d7765194328