blk-mq: export setting request completion state

This is preparing for drivers that want to directly alter the state of
their requests. No functional change here.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>