pf_purge_thread() breaks up the work of iterating all states (in
pf_purge_expired_states()) and tracks progress in the idx variable.
If multiple vnets exist this results in pf_purge_thread() only calling
pf_purge_expired_states() for part of the states (the first part of the
first vnet, second part of the second vnet and so on).
Combined with the mark-and-sweep approach to cleaning up old rules (in
V_pf_unlinked_rules) that resulted in pf freeing rules that were still
referenced by states. This in turn caused panics when pf_state_expires()
encounters that state and attempts to access the rule.
We need to track the progress per vnet, not globally, so idx is moved
into a per-vnet V_pf_purge_idx.
PR: 219251
Sponsored by: Hackathon Essen 2017
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