[Fizz] prevent reentrant finishedTask from calling completeAll multiple times (#36287)

It is possible for the fallback tasks from a Suspense boundary to
trigger an early `completeAll` call which is later repeated due to
`finishedTask` reentrancy. For node.js in particular this might be
problematic since we invoke a callback on each `completeAll` call but in
general it just isn't the right semantics since the call is running
slightly earlier than the completion of the last `finishedTask`
invocation. This change ensures that any reentrant `finishedTask` calls
(due to soft aborting fallback tasks) omit the `completeAll` call by
temporarily incrementing the total pending tasks.