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: NChris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -235,8 +235,8 @@ static int verify_parent_transid(struct extent_io_tree *io_tree,
 	       (unsigned long long)parent_transid,
 	       (unsigned long long)btrfs_header_generation(eb));
 	ret = 1;
-out:
 	clear_extent_buffer_uptodate(io_tree, eb);
+out:
 	unlock_extent(io_tree, eb->start, eb->start + eb->len - 1,
 		      GFP_NOFS);
 	return ret;

fs/btrfs/extent_io.c

@@ -2647,6 +2647,8 @@ static inline struct page *extent_buffer_page(struct extent_buffer *eb,
 		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 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree,
 			      GFP_NOFS);
 	for (i = 0; i < num_pages; i++) {
 		page = extent_buffer_page(eb, i);
-		ClearPageUptodate(page);
+		if (page)
+			ClearPageUptodate(page);
 	}
 	return 0;
 }