ipv6: Annotate change of locking mechanism for np->opt

follows up commit 45f6fad8 ("ipv6: add complete rcu protection around
np->opt") which added mixed rcu/refcount protection to np->opt.

Given the current implementation of rcu_pointer_handoff(), this has no
effect at runtime.

Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>