kernel - Handle spinlock indefinite wait edge case

kernel - Handle spinlock indefinite wait edge case

* The spinlock exclusive priority mechanism can cause an indefinite
  wait situation for shared locks to arise when a large number of cpu
  cores are cycling the same spinlock both shared and exclusive.

  This situation just won't happen for any real workload, but it
  can come up in benchmarks.

* Introduce a quick hack to ensure that this situation does not lead
  to a panic.  The exclusive priority mechanism is ignored once a
  shared spinlock has spun for greater than one second.