It looks like rcutree_report_cpu_starting() has been introduced
448e9f34d91d1a4799fdb06a93c2c24b34b6fd9d ("rcu: Standardize explicit
CPU-hotplug calls") which is in v6.7.
For MIPS, it would like an adequate fix would be to
's/rcutree_report_cpu_starting/rcu_cpu_starting/' for the 6.1 and 6.6
branches.
Stefan, do you agree?