rtnetlink: RCU-annotate both dimensions of rtnl_msg_handlers

We use rcu_assign_pointer to assign both the table and the entries,
but the entries are not marked as __rcu. This generates sparse
warnings.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>