block: Add a wrapper for setting minimum request size without a queue

Introduce blk_limits_io_min() and make blk_queue_io_min() call it.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>