block: factor out requeue handling from dispatch code

Factor out the requeue handling from the dispatch code, this will make
subsequent addition of different requeueing schemes easier.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>