sctp: move sk_route_caps check and set into sctp_outq_flush_transports

The sk's sk_route_caps is set in sctp_packet_config, and later it
only needs to change when traversing the transport_list in a loop,
as the dst might be changed in the tx path.

So move sk_route_caps check and set into sctp_outq_flush_transports
from sctp_packet_transmit. This also fixes a dst leak reported by
Chen Yi:

  https://bugzilla.kernel.org/show_bug.cgi?id=212227

As calling sk_setup_caps() in sctp_packet_transmit may also set the
sk_route_caps for the ctrl sock in a netns. When the netns is being
deleted, the ctrl sock's releasing is later than dst dev's deleting,
which will cause this dev's deleting to hang and dmesg error occurs:

  unregister_netdevice: waiting for xxx to become free. Usage count = 1
Reported-by: Chen Yi <yiche@redhat.com>
Fixes: bcd623d8 ("sctp: call sk_setup_caps in sctp_packet_transmit instead")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/output.c

@@ -584,13 +584,6 @@ int sctp_packet_transmit(struct sctp_packet *packet, gfp_t gfp)
 		goto out;
 	}
 
-	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) {

net/sctp/outqueue.c

@@ -1135,6 +1135,7 @@ static void sctp_outq_flush_data(struct sctp_flush_ctx *ctx,
 
 static void sctp_outq_flush_transports(struct sctp_flush_ctx *ctx)
 {
+	struct sock *sk = ctx->asoc->base.sk;
 	struct list_head *ltransport;
 	struct sctp_packet *packet;
 	struct sctp_transport *t;
@@ -1144,6 +1145,12 @@ static void sctp_outq_flush_transports(struct sctp_flush_ctx *ctx)
 		t = list_entry(ltransport, struct sctp_transport, send_ready);
 		packet = &t->packet;
 		if (!sctp_packet_empty(packet)) {
+			rcu_read_lock();
+			if (t->dst && __sk_dst_get(sk) != t->dst) {
+				dst_hold(t->dst);
+				sk_setup_caps(sk, t->dst);
+			}
+			rcu_read_unlock();
 			error = sctp_packet_transmit(packet, ctx->gfp);
 			if (error < 0)
 				ctx->q->asoc->base.sk->sk_err = -error;