btrfs: convert super block writes to folio in write_dev_supers()

This is a direct conversion from pages to folios, assuming single page
folio. Also removes some calls to obsolete APIs and some hidden calls to
compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>