smp-urgent-2020-02-09

 - Make the UP version of smp_call_function_single() match SMP semantics
   when called for a not available CPU.  Instead of emitting a warning and
   assuming that the function call target is CPU0, return a proper error
   code like the SMP version does.

 - Remove a superfluous check in smp_call_function_many_cond()