BZ #1151439: Set MTU for VXLAN and GRE to -50 of MTU of physint

https://bugzilla.redhat.com/show_bug.cgi?id=1151439

Actually compute the MTU instead of just returning the given value.
Merge pull request #394 from bcrochet/mtu50

BZ #1151439: Set MTU for VXLAN and GRE to -50 of MTU of physint