bpf: don't move an already-detached bpf_if in bpf_vmove

[guest-seuros]

Summary

if_vmove() unconditionally called bpf_vmove(), even after bpfdetach()
has replaced if_bpf with the dead_bpf_if sentinel. bpf_vmove() then
treated the sentinel as a live bpf_if and corrupted its list linkage,
panicking under QUEUE_MACRO_DEBUG on epair vnet teardown.

panic: bpf_vmove: Bad link elm ... next->prev != elm

Exported the sentinel via bpf_dead_if() so bpf_vmove() can recognize it
and skip it.

MFC: 1 week

Test Plan

1. Create a VNET jail and give it one peer of an epair(4):
   
   jail -c name=t vnet persist ifconfig epair0 create ifconfig epair0a vnet t

2. Inside the jail, bring the interface up and generate some traffic (anything that attaches bpf works dhclient, tcpdump -i epair0a, ect.)

3. Destroy the jail:
   
   jail -r t

I was configuring forgejo, blackship and a test runner.. Happens in current and 15.1

[glebius]

How can I reproduce the problem?

[guest-seuros]

I was writing the instructions in the test plan.

I have a blackship manifest that reproduce it 100%

Btw the system has WITNESS , INVARIANTS enabled.