Btrfs: fix buffer leak in btrfs_next_old_leaf

When calling btrfs_next_old_leaf, we were leaking an extent buffer in the
rare case of using the deadlock avoidance code needed for the tree mod log.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

fs/btrfs/ctree.c

@@ -5127,6 +5127,7 @@ again:
 				 * locked. To solve this situation, we give up
 				 * on our lock and cycle.
 				 */
+				free_extent_buffer(next);
 				btrfs_release_path(path);
 				cond_resched();
 				goto again;