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.
Actually, from poking around in IRB, I'm not sure that trailing comma is syntactically valid. In any case, it's unnecessary and contrary to existing practice, so it should be removed even if it's not a syntax error here.