linux: Initialize per-VNET state earlier

[markj]

Summary

Make sure that linux_ifnet_departure() can't run after
linux_ifnet_vnet_uninit() has destroyed the per-VNET unit number
allocator. Otherwise, when ifc_detach_cloner() destroys instances of an
interface type, it might be running too late. Most cloner-based drivers
tear themselves down during SI_SUB_PSEUDO or SI_SUB_PROTO_IF.

[pouria]

I was trying to do the same thing with PROTO_IF + SI_ORDER_FIRST on VNET_SYSUNINIT. But this version looks better to me. of course it depends on if_epair.