[PATCH 00/12 V2] target: fix cmd plugging and completion

The following patches made over Martin's 5.12 branches fix two
issues:

1. target_core_iblock plugs and unplugs the queue for every
command. To handle this issue and handle an issue that
vhost-scsi and loop were avoiding by adding their own workqueue,
I added a new submission workqueue to LIO. Drivers can pass cmds
to it, and we can then submit batches of cmds.

2. vhost-scsi and loop on the submission side were doing a work
per cmd and on the lio completion side it was doing a work per
cmd. The cap on running works is 512 (max_active) and so we can
end up end up using a lot of threads when submissions start blocking
because they hit the block tag limit or the completion side blocks
trying to send the cmd. In this patchset I just use a cmd list
per session to avoid abusing the workueue layer.

The combined patchset fixes a major perf issue we've been hitting
where IOPs is stuck at 230K when running:

    fio --filename=/dev/sda  --direct=1 --rw=randrw --bs=4k
    --ioengine=libaio --iodepth=128  --numjobs=8 --time_based
    --group_reporting --runtime=60

The patches in this set get me to 350K when using devices that
have native IOPs of around 400-500K.

Note that 5.12 has some interrupt changes that my patches
collide with. Martin's 5.12 branches had the changes so I
based my patches on that.

V2:
- Fix up container_of use coding style
- Handle offlist review comment from Laurence where with the
original code and my patches we can hit a bug where the cmd
times out, LIO starts up the TMR code, but it misses the cmd
because it's on the workqueue.
- Made the work per device work instead of session to handle
the previous issue and so if one dev hits some issue it sleeps on,
it won't block other devices.