tracing: Add per_cpu directory into tracing instances

Add the per_cpu directory to the created tracing instances:

  cd /sys/kernel/debug/tracing/instances
  mkdir foo
  ls foo/per_cpu/cpu0
buffer_size_kb	snapshot_raw  trace	  trace_pipe_raw
snapshot	stats	      trace_pipe

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>