perf sched: Make idle thread and comm/pid names more consistent

Peter noticed that we have 3 ways of referring to the idle thread:

 [idle]:0
 swapper:0
 swapper-0

Standardize on 'swapper:0'.

Reported-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>