sctp: get sock from transport in sctp_transport_update_pmtu

This patch is almost to revert commit 02f3d4ce9e81 ("sctp: Adjust PMTU
updates to accomodate route invalidation."). As t->asoc can't be NULL
in sctp_transport_update_pmtu, it could get sk from asoc, and no need
to pass sk into that function.

It is also to remove some duplicated codes from that function.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sctp/output.c b/net/sctp/output.c
index ec4d50a..1409a87 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -105,10 +105,10 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
 	if (!sctp_transport_dst_check(tp)) {
 		sctp_transport_route(tp, NULL, sctp_sk(sk));
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
-			sctp_assoc_sync_pmtu(sk, asoc);
+			sctp_assoc_sync_pmtu(asoc);
 	} else if (!sctp_transport_pmtu_check(tp)) {
 		if (asoc->param_flags & SPP_PMTUD_ENABLE)
-			sctp_assoc_sync_pmtu(sk, asoc);
+			sctp_assoc_sync_pmtu(asoc);
 	}
 
 	/* If there a is a prepend chunk stick it on the list before