btrfs: only unlock block in verify_parent_transid if we locked it

This is a regression from my patch a26e8c9f, we
need to only unlock the block if we were the one who locked it.  Otherwise this
will trip BUG_ON()'s in locking.c  Thanks,

cc: stable@vger.kernel.org
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/disk-io.c

@@ -369,6 +369,7 @@ static int verify_parent_transid(struct extent_io_tree *io_tree,
 out:
 	unlock_extent_cached(io_tree, eb->start, eb->start + eb->len - 1,
 			     &cached_state, GFP_NOFS);
+	if (need_lock)
 		btrfs_tree_read_unlock_blocking(eb);
 	return ret;
 }