Reimplement the wait_queue and schedule() APIs.

[markj]

Summary

This is mostly identical to the changes I made for drm-next.

I'm not quite prepared to commit this yet, as I'm still testing
on drm-next and want to go over the changes once more.

Test Plan

Tested i915 and amdgpu with drm-next. Only compiled-tested on HEAD.

[markj]

• Minor cleanups.

[• hselasky]

sys/compat/linuxkpi/common/src/linux_schedule.c

70 ↗

(On Diff #29044)

This should be:
else

ret = -ret;

??

[markj]

• Various bug fixes, mostly from HPS.

[• hselasky]

Test OK with OFED. Make sure kernel can be built WITH_OFED=YES. Please also add some more details about the changes to the commit message.

Thank you!

--HPS

[• hselasky]

Also make sure the word "LinuxKPI" is part of the commit message.