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

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

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