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. [1]

PR: 219472
Reported by: CTurt <ecturt@gmail.com> [1]
Reviewed by: kp (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18779