Btrfs: use 'u64' rather than 'int' to get extent's generation

We define a 'int' to get extent's generation by mistake,fix it.
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/relocation.c

@@ -3263,7 +3263,7 @@ static int add_tree_block(struct reloc_control *rc,
 	struct rb_node *rb_node;
 	u32 item_size;
 	int level = -1;
-	int generation;
+	u64 generation;
 
 	eb =  path->nodes[0];
 	item_size = btrfs_item_size_nr(eb, path->slots[0]);