We already check for started commands in all callbacks, but we should
also protect against already completed commands.  Do this by taking
the checks to common code.
Acked-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>