Don't set the MTU for loopback interfaces.

Setting the MTU for loopback has a huge performance impact on intra-container
traffic. The loopback interface is not an ethernet interface and does not need
the same limit. with this change I see a 4x speedup for loopback throughput.

Docker-DCO-1.1-Signed-off-by: Tim Hockin <thockin@google.com> (github: thockin)