interface-ip: Fix broadcast address when using /31 or /32 IPv4 addressing

A /31-addressed interface requires a broadcast address of 255.255.255.255,
because there is no room for a proper broadcast address.  Without this,
any packet destinated to the other end of the link is sent as broadcast,
which is incorrect.

For consistency with the Linux kernel, /32-addressed interfaces are
treated in the same way.

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>