perf report: Use a modifiable string for default callchain options

If the user doesn't provide options to tune his callchain output
(ie: if he uses -c without arguments) then the default value passed
in the OPT_CALLBACK_DEFAULT() macro is used.

But it's parsed later by strtok() which will replace comma separators
to a zero. This may segfault as we are using a read-only string.

Use a modifiable one instead, and also fix the "100%" default
minimum threshold value by turning it into a 0 (output every callchains)
as it was intended in the origin.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Anton Blanchard <anton@samba.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1246772361-9960-2-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>