Sync dio could be big, or may take long time in discard or in case of
IO failure.

We have prevented task hung in submit_bio_wait() and blk_execute_rq(),
so apply the same trick for prevent task hung from happening in sync dio.

Add helper of blk_io_schedule() and use io_schedule_timeout() to prevent
task hung warning.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Cc: Salman Qazi <sqazi@google.com>
Cc: Jesse Barnes <jsbarnes@google.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>