[SCTP]: Fix NULL dereference of asoc.

Commit 7cbca67c ("[IPV6]: Support
Source Address Selection API (RFC5014)") introduced NULL dereference
of asoc to sctp_v6_get_saddr in net/sctp/ipv6.c.
Pointed out by Johann Felix Soden <johfel@users.sourceforge.net>.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

include/net/sctp/structs.h

@@ -548,7 +548,8 @@ struct sctp_af {
 	struct dst_entry *(*get_dst)	(struct sctp_association *asoc,
 					 union sctp_addr *daddr,
 					 union sctp_addr *saddr);
-	void		(*get_saddr)	(struct sctp_association *asoc,
+	void		(*get_saddr)	(struct sctp_sock *sk,
+					 struct sctp_association *asoc,
 					 struct dst_entry *dst,
 					 union sctp_addr *daddr,
 					 union sctp_addr *saddr);

net/sctp/ipv6.c

@@ -299,7 +299,8 @@ static inline int sctp_v6_addr_match_len(union sctp_addr *s1,
 /* Fills in the source address(saddr) based on the destination address(daddr)
  * and asoc's bind address list.
  */
-static void sctp_v6_get_saddr(struct sctp_association *asoc,
+static void sctp_v6_get_saddr(struct sctp_sock *sk,
+			      struct sctp_association *asoc,
 			      struct dst_entry *dst,
 			      union sctp_addr *daddr,
 			      union sctp_addr *saddr)
@@ -318,7 +319,7 @@ static void sctp_v6_get_saddr(struct sctp_association *asoc,
 	if (!asoc) {
 		ipv6_dev_get_saddr(dst ? ip6_dst_idev(dst)->dev : NULL,
 				   &daddr->v6.sin6_addr,
-				   inet6_sk(asoc->base.sk)->srcprefs,
+				   inet6_sk(&sk->inet.sk)->srcprefs,
 				   &saddr->v6.sin6_addr);
 		SCTP_DEBUG_PRINTK("saddr from ipv6_get_saddr: " NIP6_FMT "\n",
 				  NIP6(saddr->v6.sin6_addr));

net/sctp/protocol.c

@@ -519,7 +519,8 @@ out:
 /* For v4, the source address is cached in the route entry(dst). So no need
  * to cache it separately and hence this is an empty routine.
  */
-static void sctp_v4_get_saddr(struct sctp_association *asoc,
+static void sctp_v4_get_saddr(struct sctp_sock *sk,
+			      struct sctp_association *asoc,
 			      struct dst_entry *dst,
 			      union sctp_addr *daddr,
 			      union sctp_addr *saddr)

net/sctp/transport.c

@@ -291,7 +291,7 @@ void sctp_transport_route(struct sctp_transport *transport,
 	if (saddr)
 		memcpy(&transport->saddr, saddr, sizeof(union sctp_addr));
 	else
-		af->get_saddr(asoc, dst, daddr, &transport->saddr);
+		af->get_saddr(opt, asoc, dst, daddr, &transport->saddr);
 
 	transport->dst = dst;
 	if ((transport->param_flags & SPP_PMTUD_DISABLE) && transport->pathmtu) {