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/net/decnet/dn_neigh.c b/net/decnet/dn_neigh.c
index a085dbc..602dade 100644
--- a/net/decnet/dn_neigh.c
+++ b/net/decnet/dn_neigh.c
@@ -391,7 +391,7 @@
 		write_lock(&neigh->lock);
 
 		neigh->used = jiffies;
-		dn_db = (struct dn_dev *)neigh->dev->dn_ptr;
+		dn_db = rcu_dereference(neigh->dev->dn_ptr);
 
 		if (!(neigh->nud_state & NUD_PERMANENT)) {
 			neigh->updated = jiffies;