rv: Fully convert enabled_monitors to use list_head as iterator

In the Linux kernel, the following vulnerability has been resolved: rv: Fully convert enabled_monitors to use list_head as iterator The callbacks in enabled_monitors_seq_ops are inconsistent. Some treat the iterator as struct rv_monitor *, while others treat the iterator as struct list_head *. This causes a wrong type cast and crashes the system as reported by Nathan. Convert everything to use struct list_head * as iterator. This also makes enabled_monitors consistent with available_monitors.