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/net/decnet/af_decnet.c b/net/decnet/af_decnet.c
index 307c322..64d9843 100644
--- a/net/decnet/af_decnet.c
+++ b/net/decnet/af_decnet.c
@@ -909,6 +909,7 @@
 	struct dn_scp *scp = DN_SK(sk);
 	int err = -EISCONN;
 	struct flowidn fld;
+	struct dst_entry *dst;
 
 	if (sock->state == SS_CONNECTED)
 		goto out;
@@ -955,10 +956,11 @@
 	fld.flowidn_proto = DNPROTO_NSP;
 	if (dn_route_output_sock(&sk->sk_dst_cache, &fld, sk, flags) < 0)
 		goto out;
-	sk->sk_route_caps = sk->sk_dst_cache->dev->features;
+	dst = __sk_dst_get(sk);
+	sk->sk_route_caps = dst->dev->features;
 	sock->state = SS_CONNECTING;
 	scp->state = DN_CI;
-	scp->segsize_loc = dst_metric_advmss(sk->sk_dst_cache);
+	scp->segsize_loc = dst_metric_advmss(dst);
 
 	dn_nsp_send_conninit(sk, NSP_CI);
 	err = -EINPROGRESS;