update comments about ktap event callback closure

tracepoint callback handler is calling with rcu_read_lock.

There have minor difference between ftrace and perf.

ftrace use ring buffer, and ring buffer disable preempt
in ring_buffer_lock_reserve, and enable preempt in
ring_buffer_unlock_commit;

perf only have rcu_read_lock protection in tracepoint handler,
it needs to check more on perf_swevent_get_recursion_context.