spl-taskq: fix task counts for delayed and cancelled tasks

Dispatched delayed tasks were not added to tasks_total, and cancelled
tasks were not removed. This notably could make tasks_total go to
UNIT64_MAX, but just generally meant the count could be wrong. So lets
not!

Sponsored-by: Klara, Inc.
Sponsored-by: Syneto
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16473