When adding a wsmux device to an existing wsmux device using

ioctl(WSMUXIO_ADD_DEVICE), two distinct locks of the same type are
acquired. Thus, witness will emit warning. Since acquiring two different
locks of the same type is harmless in this context, relax the witness
check by flagging the locks as RWL_DUPOK.

ok visa@

Reported-by: syzbot+249e483406a1f7843915@syzkaller.appspotmail.com

sys/dev/wscons/wsmux.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: wsmux.c,v 1.41 2019/02/19 07:03:29 anton Exp $	*/
+/*	$OpenBSD: wsmux.c,v 1.42 2019/02/20 17:54:34 anton Exp $	*/
 /*      $NetBSD: wsmux.c,v 1.37 2005/04/30 03:47:12 augustss Exp $      */
 
 /*
@@ -608,7 +608,7 @@ wsmux_create(const char *name, int unit)
 	if (sc == NULL)
 		return (NULL);
 	TAILQ_INIT(&sc->sc_cld);
-	rw_init(&sc->sc_lock, "wsmuxlk");
+	rw_init_flags(&sc->sc_lock, "wsmuxlk", RWL_DUPOK);
 	snprintf(sc->sc_base.me_dv.dv_xname, sizeof sc->sc_base.me_dv.dv_xname,
 		 "%s%d", name, unit);
 	sc->sc_base.me_dv.dv_unit = unit;