Otherwise, a forwarding thread may use the interface being detached. This
is a regression from 0d469d23715d, which manifests itself as a reliably
reproducible panic in in6_selecthlim(). Note that there are old bug
reports about such a panic, and I believe this change will not fix them,
as their nature is not due to a screwed up detach sequence, but due to
lack of proper epoch(9) based synchronization between the detach and
forwarding.
Reported & tested by: jhibbits
PR: 292162
Fixes: 0d469d23715d690b863787ebfa51529e1f6a9092