VNET: Revert "ifnet: make if_index global"

This reverts commit 91f44749c6feb50f39af8805dd803e860f0418f1.

Devirtualization of V_if_index and V_ifindex_table was rushed into
the tree lacking proper context, discussion, and declaration of intent,
so I'm backing it out as harmful to VNET on the following grounds:

1) The change repurposed the decades-old and stable if_index KBI for
new, unclear goals which were omitted from the commit note.

2) The change opened up a new resource exhaustion vector where any vnet
could starve the system of ifnet indices, including vnet0.

3) To circumvent the newly introduced problem of separating ifnets
belonging to different vnets from the globalized ifindex_table, the
author introduced sysctl_ifcount() which does a linear traversal over
the (potentially huge) global ifnet list just to return a simple upper
bound on existing ifnet indices.

4) The change effectively led to nonuniform ifnet index allocation
among vnets.

5) The commit note clearly stated that the patch changed the implicit
if_index ABI contract where ifnet indices were assumed to be starting
from one.  The commit note also included a correct observation that
holes in interface indices were always allowed, but failed to declare
that the userland-observable ifindex tables could now include huge
empty spans even under modest operating conditions.

6) The author had an earlier proposal in the works which did not
affect per-vnet ifnet lists (D33265) but which he abandoned without
providing the rationale behind his decision to do so, at the expense
of sacrificing the vnet isolation contract and if_index ABI / KBI.

Furthermore, the author agreed to back out his changes himself and
to follow up with a proposal for a less intrusive alternative, but
later silently declined to act.  Therefore, I decided to resolve the
status-quo by backing this out myself.  This in no way precludes a
future proposal aiming to mitigate ifnet-removal related system
crashes or panics to be accepted, provided it would not unnecessarily
compromise the goal of as strict as possible isolation between vnets.

Obtained from: github.com/glebius/FreeBSD/commits/backout-ifindex

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