sctp: call sk_setup_caps in sctp_packet_transmit instead

sk_setup_caps() was originally called in Commit 90017acc ("sctp:
Add GSO support"), as:

  "We have to refresh this in case we are xmiting to more than one
   transport at a time"

This actually happens in the loop of sctp_outq_flush_transports(),
and it shouldn't be tied to gso, so move it out of gso part and
before sctp_packet_pack().
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/output.c

@@ -508,12 +508,6 @@ static int sctp_packet_pack(struct sctp_packet *packet,
 					sizeof(struct inet6_skb_parm)));
 		skb_shinfo(head)->gso_segs = pkt_count;
 		skb_shinfo(head)->gso_size = GSO_BY_FRAGS;
-		rcu_read_lock();
-		if (skb_dst(head) != tp->dst) {
-			dst_hold(tp->dst);
-			sk_setup_caps(sk, tp->dst);
-		}
-		rcu_read_unlock();
 		goto chksum;
 	}
 
@@ -593,6 +587,13 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
 	}
 	skb_dst_set(head, dst);
 
+	rcu_read_lock();
+	if (__sk_dst_get(sk) != tp->dst) {
+		dst_hold(tp->dst);
+		sk_setup_caps(sk, tp->dst);
+	}
+	rcu_read_unlock();
+
 	/* pack up chunks */
 	pkt_count = sctp_packet_pack(packet, head, gso, gfp);
 	if (!pkt_count) {