changed so that ext hdr len code checks against buffer size instead of link mtu
changed so that ext hdr len code checks against buffer size instead of link mtu

arurke/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,arurke/contiki,arurke/contiki,bluerover/6lbr,bluerover/6lbr