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
Reported by: Randy Westlund <rwestlun@gmail.com>
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7339