block: remove bio_iov_iter_get_pages

Switch the only caller to bio_iov_iter_get_pages, and explain why it does
not have any alignment requirements.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>