ipv6: Add in6_dev_rcu().

rcu_dereference_rtnl() does not clearly tell whether the caller
is under RCU or RTNL.

Let's add in6_dev_rcu() to make it easy to remove __in6_dev_get()
in the future.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Guillaume Nault <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/addrconf.h

···
 	return rcu_dereference_rtnl(dev->ip6_ptr);
 }
 
+static inline struct inet6_dev *in6_dev_rcu(const struct net_device *dev)
+{
+	return rcu_dereference(dev->ip6_ptr);
+}
+
 static inline struct inet6_dev *__in6_dev_get_rtnl_net(const struct net_device *dev)
 {
 	return rtnl_net_dereference(dev_net(dev), dev->ip6_ptr);