let's not pretend switch(4) works with anything except ethernet interfaces.

[David Gwynne]

the code pretty obviously assumes that it only handles Ethernet packets,
so we should restrict it to IF_ETHER type interfaces.

ok?

Index: if_switch.c
===================================================================
RCS file: /cvs/src/sys/net/if_switch.c,v
retrieving revision 1.30
diff -u -p -r1.30 if_switch.c
--- if_switch.c 6 Nov 2019 03:51:26 -0000       1.30
+++ if_switch.c 9 Jul 2020 05:59:51 -0000
@@ -506,6 +506,9 @@ switch_port_add(struct switch_softc *sc,
        if ((ifs = ifunit(req->ifbr_ifsname)) == NULL)
                return (ENOENT);
 
+       if (ifs->if_type != IFT_ETHER)
+               return (EPROTONOSUPPORT);
+
        if (ifs->if_bridgeidx != 0)
                return (EBUSY);
 
@@ -517,15 +520,12 @@ switch_port_add(struct switch_softc *sc,
                        return (EBUSY);
        }
 
-       if (ifs->if_type == IFT_ETHER) {
-               if ((error = ifpromisc(ifs, 1)) != 0)
-                       return (error);
-       }
+       if ((error = ifpromisc(ifs, 1)) != 0)
+               return (error);
 
        swpo = malloc(sizeof(*swpo), M_DEVBUF, M_NOWAIT|M_ZERO);
        if (swpo == NULL) {
-               if (ifs->if_type == IFT_ETHER)
-                       ifpromisc(ifs, 0);
+               ifpromisc(ifs, 0);
                return (ENOMEM);
        }
        swpo->swpo_switch = sc;