pf: Fix panic on vnet jail shutdown with synproxy

When shutting down a vnet jail pf_shutdown() clears the remaining states, which
through pf_clear_states() calls pf_unlink_state().
For synproxy states pf_unlink_state() will send a TCP RST, which eventually
tries to schedule the pf swi in pf_send(). This means we can't remove the
software interrupt until after pf_shutdown().

MFC after: 1 week