mm: move clearing of page->mapping to page_cache_tree_delete()

Clearing of page->mapping makes sense in page_cache_tree_delete() as
well and it will help us with batching things this way.

Link: http://lkml.kernel.org/r/20171010151937.26984-6-jack@suse.czSigned-off-by: NJan Kara <jack@suse.cz>
Acked-by: NMel Gorman <mgorman@suse.de>
Reviewed-by: NAndi Kleen <ak@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -165,6 +165,9 @@ static void page_cache_tree_delete(struct address_space *mapping,
 				     workingset_update_node, mapping);
 	}
 
+	page->mapping = NULL;
+	/* Leave page->index set: truncation lookup relies upon it */
+
 	if (shadow) {
 		mapping->nrexceptional += nr;
 		/*
@@ -250,9 +253,6 @@ void __delete_from_page_cache(struct page *page, void *shadow)
 					     inode_to_wb(mapping->host));
 	}
 	page_cache_tree_delete(mapping, page, shadow);
-
-	page->mapping = NULL;
-	/* Leave page->index set: truncation lookup relies upon it */
 }
 
 static void page_cache_free_page(struct address_space *mapping,