decnet: use correct RCU API to deref sk_dst_cache field

sock->sk_dst_cache is protected by RCU, therefore we should
use __sk_dst_get() to deref it once we lock the sock.

This fixes several sparse warnings.

Cc: linux-decnet-user@lists.sourceforge.net
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/dn_route.h b/include/net/dn_route.h
index 4f7d6a1..2e9d317 100644
--- a/include/net/dn_route.h
+++ b/include/net/dn_route.h
@@ -16,7 +16,7 @@
 *******************************************************************************/
 
 extern struct sk_buff *dn_alloc_skb(struct sock *sk, int size, gfp_t pri);
-extern int dn_route_output_sock(struct dst_entry **pprt, struct flowidn *, struct sock *sk, int flags);
+extern int dn_route_output_sock(struct dst_entry __rcu **pprt, struct flowidn *, struct sock *sk, int flags);
 extern int dn_cache_dump(struct sk_buff *skb, struct netlink_callback *cb);
 extern void dn_rt_cache_flush(int delay);