btrfs: stop using write_one_page in btrfs_scratch_superblock

write_one_page is an awkward interface that expects the page locked and
->writepage to be implemented.  Replace that by zeroing the signature
bytes and synchronize the block device page using the proper bdev
helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>