mm: Convert is_page_cache_freeable to XArray

This is just a variable rename and comment change.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

mm/vmscan.c

@@ -742,12 +742,12 @@ static inline int is_page_cache_freeable(struct page *page)
 {
 	/*
 	 * A freeable page cache page is referenced only by the caller
-	 * that isolated the page, the page cache radix tree and
-	 * optional buffer heads at page->private.
+	 * that isolated the page, the page cache and optional buffer
+	 * heads at page->private.
 	 */
-	int radix_pins = PageTransHuge(page) && PageSwapCache(page) ?
+	int page_cache_pins = PageTransHuge(page) && PageSwapCache(page) ?
 		HPAGE_PMD_NR : 1;
-	return page_count(page) - page_has_private(page) == 1 + radix_pins;
+	return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
 }
 
 static int may_write_to_inode(struct inode *inode, struct scan_control *sc)