truncate: Skip known-truncated indices

If we've truncated an entire folio, we can skip over all the indices
covered by this folio.

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