There's a race where dying vnets move their interfaces back to their
original vnet, and if_epair cleanup (where deleting one interface also
deletes the other end of the epair). This is commonly triggered by the
pf tests, but also by cleanup of vnet jails.
As we've not yet been able to fix the root cause of the issue work
around the panic by not dereferencing a NULL softc in epair_qflush() and
by not re-attaching DYING interfaces.
This isn't a full fix, but makes a very common panic far less likely.
PR: 244703, 238870