iomap: Simplify is_partially_uptodate a little

Remove the unnecessary variable 'len' and fix a comment to refer to
the folio instead of the page.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>

fs/iomap/buffered-io.c

@@ -435,18 +435,17 @@ bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
 {
 	struct iomap_page *iop = to_iomap_page(folio);
 	struct inode *inode = folio->mapping->host;
-	size_t len;
 	unsigned first, last, i;
 
 	if (!iop)
 		return false;
 
-	/* Limit range to this folio */
-	len = min(folio_size(folio) - from, count);
+	/* Caller's range may extend past the end of this folio */
+	count = min(folio_size(folio) - from, count);
 
-	/* First and last blocks in range within page */
+	/* First and last blocks in range within folio */
 	first = from >> inode->i_blkbits;
-	last = (from + len - 1) >> inode->i_blkbits;
+	last = (from + count - 1) >> inode->i_blkbits;
 
 	for (i = first; i <= last; i++)
 		if (!test_bit(i, iop->uptodate))