block: check for timeout function in blk_rq_timed_out()

rq_timed_out_fn might have been unset while the request
was in flight, so we need to check for it in blk_rq_timed_out().

Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>