Update igb0 interface netmask on FreeBSD servers

Summary: Canonical IPs use /24, aliases /32 netmask.

Test Plan: Configuration per Ysul and WindRiver live config

Reviewers: DorianWinty, dereckson

Reviewed By: DorianWinty, dereckson

Differential Revision: https://devcentral.nasqueron.org/D2703