tracing: move utility functions from ftrace.h to kernel.h

Make common utility functions such as trace_printk() and
tracing_start()/tracing_stop() generally available to kernel
code.

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

diff --cc include/linux/ftrace.h
Simple merge

diff --cc include/linux/kernel.h
Simple merge