__blk_complete_request is only called from the blk-mq code, and
duplicates a lot of code from blk-mq.c.  Move it there to prepare
for better code sharing and simplifications.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NDaniel Wagner <dwagner@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>