decnet: RCU conversion and get rid of dev_base_lock

While tracking dev_base_lock users, I found decnet used it in
dnet_select_source(), but for a wrong purpose:

Writers only hold RTNL, not dev_base_lock, so readers must use RCU if
they cannot use RTNL.

Adds an rcu_head in struct dn_ifaddr and handle proper RCU management.

Adds __rcu annotation in dn_route as well.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/dst.h b/include/net/dst.h
index ffe9cb7..a5bd726 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -94,10 +94,10 @@
 	int			__use;
 	unsigned long		lastuse;
 	union {
-		struct dst_entry *next;
-		struct rtable __rcu *rt_next;
-		struct rt6_info   *rt6_next;
-		struct dn_route  *dn_next;
+		struct dst_entry	*next;
+		struct rtable __rcu	*rt_next;
+		struct rt6_info		*rt6_next;
+		struct dn_route __rcu	*dn_next;
 	};
 };