rv: Fully convert enabled_monitors to use list_head as iterator

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.

Fixes: de090d1ccae1 ("rv: Fix wrong type cast in enabled_monitors_next()")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/linux-trace-kernel/20250923002004.GA2836051@ax162/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Cc: stable@vger.kernel.org
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Link: https://lore.kernel.org/r/20251002082235.973099-1-namcao@linutronix.de
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>