Stop setting if_linkmib in vlan(4) ifnets.
There are several reasons:
- The structure being exported via IFDATA_LINKSPECIFIC doesn't appear to be a standard MIB.
- The structure being exported is private to the kernel and always has been.
- No other drivers in common use set the if_linkmib field.
- Because IFDATA_LINKSPECIFIC can be used to overwrite the linkmib structure, a privileged user could use it to corrupt internal vlan(4) state. 
Reported by: CTurt <firstname.lastname@example.org> 
Reviewed by: kp (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18779