perf pmu: Factor out scale conversion code

Move the scale factor parsing code to an own function to reuse it in an
upcoming patch.

v2: Return error in case strdup returns NULL.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170103150833.6694-2-andi@firstfloor.org


[ Keep returning -ENOMEM when strdup() fails in perf_pmu__parse_scale()/convert_scale() ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>