file-posix: Avoid aio_worker() for QEMU_AIO_FLUSH

file-posix: Avoid aio_worker() for QEMU_AIO_FLUSH

aio_worker() doesn't add anything interesting, it's only a useless
indirection. Call the handler function directly instead.

As we know that this handler function is only called from coroutine
context and the coroutine stays around until the worker thread finishes,
we can keep RawPosixAIOData on the stack.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>