Btrfs: don't check nodes for extent items

The backref code was looking at nodes as well as leaves when we tried to
populate extent item entries.  This is not good, and although we go away with it
for the most part because we'd skip where disk_bytenr != random_memory,
sometimes random_memory would match and suddenly boom.  This fixes that problem.
Thanks,
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/backref.c

@@ -988,11 +988,12 @@ static int find_parent_nodes(struct btrfs_trans_handle *trans,
 				goto out;
 		}
 		if (ref->count && ref->parent) {
-			if (extent_item_pos && !ref->inode_list) {
+			if (extent_item_pos && !ref->inode_list &&
+			    ref->level == 0) {
 				u32 bsz;
 				struct extent_buffer *eb;
 				bsz = btrfs_level_size(fs_info->extent_root,
-							info_level);
+							ref->level);
 				eb = read_tree_block(fs_info->extent_root,
 							   ref->parent, bsz, 0);
 				if (!eb || !extent_buffer_uptodate(eb)) {