kdebug: use for_each_online_cpu instead of for_each_possible_cpu

A bug found by Jonathan Corbet:

	unable create tracepoint event sys_enter_mmap on cpu 4, err: -19
	unable create tracepoint event sys_enter_mmap on cpu 5, err: -19
	unable create tracepoint event sys_enter_mmap on cpu 6, err: -19
	unable create tracepoint event sys_enter_mmap on cpu 7, err: -19
	[...]

This code is:
	for_each_possible_cpu(cpu)
		enable_tracepoint_on_cpu(cpu, &attr, call, arg, type);

The reason is we should use for_each_online_cpu, instead of for_each_possible_cpu

Thanks Corbet, he suggested this fix.

Reported-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@huawei.com>