if_ovpn: Fix memory leak in VNET during unload

Unloading if_ovpn while it's in use by other vnets causes
memory leaks and panics.
Fix this by reverting VNET_SYSUNINIT and adjusting the SI_SUB
initialization order.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54175