spl-taskq: per-taskq kstats

This exposes a variety of per-taskq stats under /proc/spl/kstat/taskq,
one file per taskq, named for the taskq name.instance.

These include a small amount of info about the taskq config, the current
state of the threads and queues, and various counters for thread and
queue activity since the taskq was created.

To assist with decrementing queue size counters, the list an entry is on
is encoded in spare bits in the entry flags.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Syneto
Closes #16171