[$] Warning about WARN_ON()

Kernel developers, like conscientious developers for many projects, willoften include checks in the code for conditions that are never expected tooccur, but which would indicate a serious problem should that expectationturn out to be incorrect. For years, developers have been encouraged (toput it politely) to avoid using assertions that crash the machine for suchconditions unless there is truly no alternative. Increasingly, though, useof the kernel's WARN_ON() family of macros, which developers weretold to use instead, is also being discouraged.