Allow the path MTU to grow up to the outgoing interface MTU.

MFC after: 3 days

sys/netinet/sctp_timer.c

@@ -1492,6 +1492,8 @@ sctp_pathmtu_timer(struct sctp_inpcb *inp,
 #endif
 			if (mtu > next_mtu) {
 				net->mtu = next_mtu;
+			} else {
+				net->mtu = mtu;
 			}
 		}
 	}