blk-mq: support at_head inserations for blk_execute_rq

This is neede for proper SG_IO operation as well as various uses of
blk_execute_rq from the SCSI midlayer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>