Make sure that iocb->aiocb is NULL'ed when cancelling.
Fix a potential use-after-free by removing the bottom half and enqueuing
the completion directly.
Fixes: 38f4ac65ac88 ("hw/nvme: reimplement flush to allow cancellation")
Reviewed-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
3dbc1708ea