filemap: Convert filemap_read() to use a folio

We know the pagevec always contains folios, but use page_folio() anyway
instead of casting.  Removes a few calls to legacy functions.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>