A network's MTU is now only valid if it is the minimum value
allowed based on the IP version of the associated subnets,
68 for IPv4 and 1280 for IPv6.
This minimum is now enforced in the following ways:
1) When a subnet is associated with a network, validate
the MTU is large enough for the IP version. Not only
would the subnet be unusable if it was allowed, but the
Linux kernel can fail adding addresses and configuring
network settings like the MTU.
2) When a network MTU is changed, validate the MTU is large
enough for any currently associated subnets. Allowing a
smaller MTU would render any existing subnets unusable.
Closes-bug: #1988069
Change-Id: Ia4017a8737f9a7c63945df546c8a7243b2673ceb
88ce859b56