Fix PMTU compatibility with MX

    MX considers Eth header size of 18 to calculate PMTU. Vrouter uses 14, changed it to 18.

    For some packets (originated with linklocal address of the VM), the PMTU messages are not sent back because
    we were using route-lookup to deliver the packets. Changed this to a direct VIF TX. Need to include VLAN
    support for this. I'm working on a separate PR to include VLAN support for IPv6 (requires changes in multiple
    places). Will address this also along with the other changes

    Address code review comment from Raja ( added back pkt_set_network_header(pkt, pkt->vp_data);)

    Closes-bug: 1382647

Change-Id: Ie76ce3d6a08330ff0e30886c7aa37d4bffa5e996