perf tools: Unify swapper tasks naming

In perf tools, we hardcode the pid 0 cmdline resolving to
"idle" because the init task is not included in the COMM
events.

But the idle tasks secondary cpus are resolved into their
"init" name through the COMM events.

We have then such strange result in perf report (ditto with
trace):

    19.66%       init    [kernel]          [k] acpi_idle_enter_c1
    17.32%       [idle]  [kernel]          [k] acpi_idle_enter_c1

It's then better to unify the swapper tasks into a single init
name.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1251693921-6579-3-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>