sctp: check cached dst before using it

dst_check() will take care of SA (and obsolete field), hence
IPsec rekeying scenario is taken into account.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Vlad Yaseivch <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/output.c b/net/sctp/output.c
index 817174e..8fc4dcd 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -377,9 +377,7 @@
 	 */
 	skb_set_owner_w(nskb, sk);
 
-	/* The 'obsolete' field of dst is set to 2 when a dst is freed. */
-	if (!dst || (dst->obsolete > 1)) {
-		dst_release(dst);
+	if (!sctp_transport_dst_check(tp)) {
 		sctp_transport_route(tp, NULL, sctp_sk(sk));
 		if (asoc && (asoc->param_flags & SPP_PMTUD_ENABLE)) {
 			sctp_assoc_sync_pmtu(asoc);