Stop setting if_linkmib in vlan(4) ifnets.

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