blk-mq: don't time out requests again that are in the timeout handler

We can currently call the timeout handler again on a request that has
already been handed over to the timeout handler.  Prevent that with a new
flag.

Fixes: 12f5b931

 ("blk-mq: Remove generation seqeunce")
Reported-by: Andrew Randrianasulu <randrianasulu@gmail.com>
Tested-by: Andrew Randrianasulu <randrianasulu@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>