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.