let vlan(4) mtu be limited by the parents hardmtu instead of current mtu

[David Gwynne]

this lets you have networks on the "native" vlan on an interface
at 1500, while setting a child vlan interfaces mtu to jumbos.

ok?

Index: if_vlan.c
===================================================================
RCS file: /cvs/src/sys/net/if_vlan.c,v
retrieving revision 1.108
diff -u -p -r1.108 if_vlan.c
--- if_vlan.c   12 Jul 2014 18:44:22 -0000      1.108
+++ if_vlan.c   19 Aug 2014 23:52:15 -0000
@@ -528,9 +528,9 @@ vlan_ioctl(struct ifnet *ifp, u_long cmd
        case SIOCSIFMTU:
                if (ifv->ifv_p != NULL) {
                        if (ifv->ifv_p->if_capabilities & IFCAP_VLAN_MTU)
-                               p_mtu = ifv->ifv_p->if_mtu;
+                               p_mtu = ifv->ifv_p->if_hardmtu;
                        else
-                               p_mtu = ifv->ifv_p->if_mtu - EVL_ENCAPLEN;
+                               p_mtu = ifv->ifv_p->if_hardmtu - EVL_ENCAPLEN;
                        
                        if (ifr->ifr_mtu > p_mtu || ifr->ifr_mtu < ETHERMIN)
                                error = EINVAL;