block: move bio_iov_iter_get_bdev_pages to block/fops.c

Keep bio_iov_iter_get_bdev_pages local with the callers, as blindly
looking at the bdev logical block size is often not the best idea
unless on a block device.

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>