perf expr: Add d_ratio operation

perf expr: Add d_ratio operation

d_ratio avoids division by 0 yielding infinity, such as when a counter
doesn't get scheduled. An example usage is:

  {
      "BriefDescription": "DCache L1 misses",
      "MetricExpr": "d_ratio(MEM_LOAD_RETIRED.L1_MISS, MEM_LOAD_RETIRED.L1_HIT + MEM_LOAD_RETIRED.L1_MISS + MEM_LOAD_RETIRED.FB_HIT)",
      "MetricGroup": "DCache;DCache_L1",
      "MetricName": "DCache_L1_Miss",
      "ScaleUnit": "100%",
  }

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200610235823.52557-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>