Btrfs: Fix verify_parent_transid

It was incorrectly clearing the up to date flag on the buffer even
when the buffer properly verified.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index e3547a9..964ec16 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -2647,6 +2647,8 @@
 		return eb->first_page;
 	i += eb->start >> PAGE_CACHE_SHIFT;
 	mapping = eb->first_page->mapping;
+	if (!mapping)
+		return NULL;
 	read_lock_irq(&mapping->tree_lock);
 	p = radix_tree_lookup(&mapping->page_tree, i);
 	read_unlock_irq(&mapping->tree_lock);
@@ -2908,7 +2910,8 @@
 			      GFP_NOFS);
 	for (i = 0; i < num_pages; i++) {
 		page = extent_buffer_page(eb, i);
-		ClearPageUptodate(page);
+		if (page)
+			ClearPageUptodate(page);
 	}
 	return 0;
 }