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/input.c b/net/sctp/input.c
index 2a28ab2..0e06a27 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -401,10 +401,10 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 
 	if (t->param_flags & SPP_PMTUD_ENABLE) {
 		/* Update transports view of the MTU */
-		sctp_transport_update_pmtu(sk, t, pmtu);
+		sctp_transport_update_pmtu(t, pmtu);
 
 		/* Update association pmtu. */
-		sctp_assoc_sync_pmtu(sk, asoc);
+		sctp_assoc_sync_pmtu(asoc);
 	}
 
 	/* Retransmit with the new pmtu setting.