The IPv6 header is twice the size of the IPv4 header, 40 vs 20
bytes, but the tunnel overhead constants are static, only
accounting for an IPv4 header in all cases. In order to be
correct it needs to treat the tunnel overhead different from
the IP overhead at L3.
This required removing the 20 byte IP overhead from the tunnel
type overhead constants and creating a new option,
ml2.overlay_ip_version, in order for the server to know which
version will be used, since it calculates the MTU for the network.
A version mis-match will now cause a tunnel sync to fail on
the server.
Moved all MTU tests to a common location to remove duplication.
DocImpact
Change-Id: Ia2546c4c71ff48b9fe2817fbad22b1fbf85f325b
Closes-bug: #1584940
51a697817d