dm delay: for short delays, use kthread instead of timers and wq

DM delay's current design of using timers and wq to realize the delays
is insufficient for delays below ~50ms.

This commit enhances the design to use a kthread to flush the expired
delays, trading some CPU time (in some cases) for better delay
accuracy and delays closer to what the user requested for smaller
delays. The new design is chosen as long as all the delays are below
50ms.

Since bios can't be completed in interrupt context using a kthread
is probably the most reasonable way to approach this.

Testing with
echo "0 2097152 zero" | dmsetup create dm-zeros
for i in $(seq 0 20);
do
  echo "0 2097152 delay /dev/mapper/dm-zeros 0 $i" | dmsetup create dm-delay-${i}ms;
done

Some performance numbers for comparison, on beaglebone black (single
core) CONFIG_HZ_1000=y:

fio --name=1msread --rw=randread --bs=4k --runtime=60 --time_based \
    --filename=/dev/mapper/dm-delay-1ms
Theoretical maximum: 1000 IOPS
Previous: 250 IOPS
Kthread: 500 IOPS

fio --name=10msread --rw=randread --bs=4k --runtime=60 --time_based \
    --filename=/dev/mapper/dm-delay-10ms
Theoretical maximum: 100 IOPS
Previous: 45 IOPS
Kthread: 50 IOPS

fio --name=1mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 \
    --time_based --filename=/dev/mapper/dm-delay-1ms
Theoretical maximum: 1000 IOPS
Previous: 498 IOPS
Kthread: 1000 IOPS

fio --name=10mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 \
    --time_based --filename=/dev/mapper/dm-delay-10ms
Theoretical maximum: 100 IOPS
Previous: 90 IOPS
Kthread: 100 IOPS

(This one is just to prove the new design isn't impacting throughput,
not really about delays):
fio --name=10mswriteasync --rw=randwrite --direct=1 --bs=4k \
    --runtime=60 --time_based --filename=/dev/mapper/dm-delay-10ms \
    --numjobs=32 --iodepth=64 --ioengine=libaio --group_reporting
Previous: 13.3k IOPS
Kthread: 13.3k IOPS

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
[Harshit: kthread_create error handling fix in delay_ctr]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>