The aio_return function returns the final return status for the
asynchronous I/O request with control block pointed to by aiocbp.
This function should be called only once for any given request, after
aio_error(2) returns something other than EINPROGRESS.
If the asynchronous I/O operation has completed, this function returns the value
that would have been returned in case of a synchronous read,
write, or fsync request. Otherwise the return value is
undefined. On error, the error value is returned.