nvme: don't abort completed request in nvme_cancel_request

nvme: don't abort completed request in nvme_cancel_request

Before aborting in-flight requests, all IO queues and their interrupts
have been shutdown. However, request's completion function may not be
done yet because it can be scheduled to run via IPI.

So don't abort one request if it is marked as completed, otherwise
we may abort one normal completed request.

Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Keith Busch <keith.busch@intel.com>
Cc: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>