vnet: Make vnet_sys[un]init() static

These two functions are intended to be used only when allocating or
destroying vnet instances.

No functional change intended.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37955

(cherry picked from commit fb9b76e05247e014f5faa554161028f6aa409bfd)
(cherry picked from commit 7b6d9cae9760152f0994d0825ee49cb5b5e204dc)