block: change return type to void

Now that all the callers of __blkdev_issue_discard() have been changed
to ignore its return value, change its return type from int to void.

Signed-off-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>