MFC 303406,303501: Fix panic when using aio_fsync().

303406:
Adjust tests in fsync job scheduling loop to reduce indentation.

303501:
Fix locking issues with aio_fsync().

• Use correct lock in aio_cancel_sync when dequeueing job.
• Add _locked variants of aio_set/clear_cancel_function and use those to avoid lock recursion when adding and removing fsync jobs to the per-process sync queue.
• While here, add a basic test for aio_fsync().

PR: 211390
Approved by: re (kib)