filemap: Add filemap_get_folios()

This is the equivalent of find_get_pages() but fills a folio_batch
instead of an array of pages.

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