A small set of fixes for the timers core:
- Fix the warning condition in __run_timers() which does not take into
account, that a CPU base (especially the deferrable base) has never a
timer armed on it and therefore the next_expiry value can become stale.
- Replace a WARN_ON() in the NOHZ code with a WARN_ON_ONCE() to prevent
endless spam in dmesg.
- Remove the double star from a comment which is not meant to be in
kernel-doc format.