ext4: Convert mpage_release_unused_pages() to use filemap_get_folios()

If the folio is large, it may overlap the beginning or end of the
unused range.  If it does, we need to avoid invalidating it.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>