sound fixes for 4.15-rc8

A collection of the last-minute small PCM fixes:
- A workaround for the recent regression wrt PulseAudio
- Removal of spurious WARN_ON() that is triggered by syzkaller
- Fixes for aloop, hardening racy accesses
- Fixes in PCM OSS emulation wrt the unabortable loops that may cause
  RCU stall
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