sound fixes for 4.17-rc1

The main purpose of this pull request is a fix for a regression
in the recent PCM OSS emulation code that may lead to RCU stall.
Since syzkaller hits this too often, I send the pull request now
with a minimal collection.  Possibly another pull request may
follow before RC1.

The other fixes here are for USB-audio class 2 and 3 to improve
the parser for the clock descriptors.  These are rather cleanups
but good for security, too.

Last but not least, another included fix is the trivial one to
remove superfluous WARN_ON() that annoyed syzbot.
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