to_shrink_list(): call only if refcount is 0

The only thing it does if refcount is not zero is d_lru_del(); no
point, IMO, seeing that plain dput() does nothing of that sort...

Note that 2 of 3 current callers are guaranteed that refcount is 0.

Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>