switchmode  - not update_mac when destination mac is other veth



In switchmode, the destination mac address should be the mac address
of container mac address not the ipop mac address. update_mac changes
destination mac address to ipop mac address. Then the frame does not
reach to container. More accurate implementation would be tap device
keeping ARP table or query O/S whether certain mac address is in
network. But, we simply check src_mac == dest_mac. If it is the same
we assume both mac address is remote ipop mac, thus update it to
local one, otherwise, we assume it comes from the remote container