LinuxKPI: extend kfifo to be usable by a driver

[bz]

Summary

Implement some basic needs for an actual driver using kfifo.

MFC after: 2 weeks

Test Plan

I had this in user space initially and tested it and it seemed to
work as expected.

[• hselasky]

sys/compat/linuxkpi/common/include/linux/kfifo.h

91

Is there a chance of counters wrapping around?

Where is first decremented?

[bz]

sys/compat/linuxkpi/common/include/linux/kfifo.h

91

First is decremented right below there if it rolls over. It's a FIFO--first and last just move forward (or wrap around).

[• hselasky]

Now I see. Looks good to me.

[bz]

I've done a build against drm-kmod master but I'll wait to hear back from @manu or @wulf on whether it's okay for them too.

[wulf]

It builds for me on master, 5.10-lts and 5.4-lts branches