block: use __bio_add_page in bio_copy_kern

block: use __bio_add_page in bio_copy_kern

Since the bio is allocated with the exact number of pages needed via
blk_rq_map_bio_alloc(), and the loop iterates exactly that many times,
bio_add_page() cannot fail due to insufficient space.  Switch to
__bio_add_page() and remove the dead error handling code.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>