The aio_fsync function does a sync on all outstanding asynchronous I/O
operations associated with aiocbp->aio_fildes.
More precisely, if op is O_SYNC, then all currently queued I/O operations
shall be completed as if by a call of fsync(2), and if op is
O_DSYNC, this call is the asynchronous analog of fdatasync(2). Note
that this is a request only - this call does not wait for I/O completion.
Apart from aio_fildes the only field in the structure pointed to by
aiocbp that is used by this call is the aio_sigevent field (a
struct sigevent) that indicates the desired type of asynchronous notification
at completion. All other fields are ignored.