Fix vlan creation for the older ifconfig(8) binaries.

Approved by: so
Security: EN-21:20.vlan
Reported by: allanjude
MFC after: immediately

(cherry picked from commit afbb64f1d85b7d8c2938031c3567946b5d10da4f)
(cherry picked from commit 9abc85d17d0538704641fe2c4d9da1b666f86e4e)