Kernel: Prevent recursive calls into the scheduler

Upon leaving a critical section (such as a SpinLock) we need to
check if we're already asynchronously invoking the Scheduler.
Otherwise we might end up triggering another context switch
as soon as leaving the scheduler lock.

Fixes #2883

Kernel/Forward.h

@@ -57,6 +57,7 @@ class Range;
 class RangeAllocator;
 class Region;
 class Scheduler;
+class SchedulerPerProcessorData;
 class SharedBuffer;
 class Socket;
 template<typename BaseType>