Don't auto-set broadcast unless subnet larger than /31
Don't auto-set broadcast unless subnet larger than /31

Since [vishvananda/netlink#248](https://github.com/vishvananda/netlink/pull/248), adding an address automatically sets the broadcast if the broadcast address was not specified. This is undesirable when adding an IP with a prefixlen of /31 or /32. (Additional details in the issues linked below.)

This changes the behavior so that the broadcast is only automatically set if the prefixlen is /30 or larger.

Issue reported in:

- https://github.com/vishvananda/netlink/issues/329
- https://github.com/vishvananda/netlink/issues/471

See also:

- [RFC 3021](http://tools.ietf.org/html/rfc3021)

Alternatives to this PR:

A. https://github.com/vishvananda/netlink/issues/472 - Adds `AddrAddWithoutCalculatedBroadcast`.
B. https://github.com/jjastrze-ovh/netlink/commit/9a85a619d2f74114626ca04b83b56f5a84f711df - Breaking change to make auto-setting the broadcast address an opt-in feature.
C. already works - Suppress setting the broadcast when addr's broadcast address is set to `0.0.0.0`. (This works today, but I'm not sure the behavior can be relied upon as a public API.)

vishvananda/netlink,paravmellanox/netlink