ifconfig: Fix default netmask calculation
Approved by: so
Security: FreeBSD-EN-24:14.ifconfig
Reported by: phk
Reviewed by: emaste, kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45570
(cherry picked from commit 8a9f0fa42b1c6cffd45459bb552e138083b00369)
(cherry picked from commit 048ad7a9ef9fe15368ff287db5c705c8163f4e1c)