ftrace: enable trace_printk by default

Impact: have the ftrace_printk enabled on startup

It is confusing to have to "echo trace_printk > /debug/tracing/iter_ctrl"
after adding ftrace_printk in the kernel.

Currently the trace_printk is set to off by default. ftrace_printk
should only be in open kernel code when used for debugging, and thus
it should be enabled by default.

It may also be used to record data within a tracer, but those ftrace_printks
should be within wrappers that are either enabled by trace_points or
have a variable protecting the code path from being entered when the
tracer is disabled.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>