We can get to pfi_kkif_remove_if_unref() via at least two distinct paths:

• when the struct ifnet is removed, via pfi_detach_ifnet_event()
• when a rule referencing us is removed, via pfi_kkif_unref().

These two events can race against each other, leading us to free this kif twice.
That leads to loop in V_pfi_unlinked_kifs, and an eventual deadlock.

Avoid this by making sure we only ever insert the kif into V_pfi_unlinked_kifs
once. If we don't find it in V_pfi_ifs it's already been removed. Check that it
exists in V_pfi_unlinked_kifs (for INVARIANTS).

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