mm: Add split_folio_to_list()

This is a convenience function; split_huge_page_to_list() can take
any page in a folio (and does so on purpose because that page will
be the one which keeps the refcount).  But it's convenient for the
callers to pass the folio instead of the first page in the folio.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>