tracing: Make sure trace_printk() can output as soon as it can be used

In the Linux kernel, the following vulnerability has been resolved:

tracing: Make sure trace_printk() can output as soon as it can be used

Currently trace_printk() can be used as soon as early_trace_init() is

called from start_kernel(). But if a crash happens, and

"ftrace_dump_on_oops" is set on the kernel command line, all you get will

be:

[ 0.456075] <idle>-0 0dN.2. 347519us : Unknown type 6

[ 0.456075] <idle>-0 0dN.2. 353141us : Unknown type 6

[ 0.456075] <idle>-0 0dN.2. 358684us : Unknown type 6

This is because the trace_printk() event (type 6) hasn't been registered

yet. That gets done via an early_initcall(), which may be early, but not

early enough.

Instead of registering the trace_printk() event (and other ftrace events,

which are not trace events) via an early_initcall(), have them registered at

the same time that trace_printk() can be used. This way, if there is a

crash before early_initcall(), then the trace_printk()s will actually be

useful.