tracing: give easy way to clear trace buffer

There is currently no easy way to clear the trace buffer. Currently
the only way is to change the current tracer.

This patch lets the user clear the trace buffer by simply writing
into the trace files.

 echo > /debug/tracing/trace

or to clear a single cpu (i.e. for CPU 1):

 echo > /debug/tracing/per_cpu/cpu1/trace

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