Rework general approach to locking and working with audio worker thread:
- Use flags to signal requested worker action
- Fix submitted buffer calculations to avoid samples starvation
- Protect buffer pointers with locks to fix race condition between callback
and audio worker thread
- Remove unnecessary vchi_service_use
- Do not use lock to serialize VCHI requests since only one thread issues them now
- Fix unloading signaling per hselasky@ suggestion
- Add output to detect inconsistent callback data caused by possible firmware bug
https://github.com/raspberrypi/firmware/issues/696
- Add stats/debug sysctls to troubleshoot possible bugs
PR: 213687, 205979, 215194
MFC after: 1 week