perf top: Save and display the drop count stats

perf top: Save and display the drop count stats

Add drop count to 'perf top' headers:

  # perf top --stdio
   PerfTop:    3549 irqs/sec  kernel:51.8%  exact: 100.0% lost: 0/0 drop: 0/0 [4000Hz cycles:ppp],  (all, 8 CPUs)

  # perf top
  Samples: 0  of event 'cycles:ppp', 4000 Hz, Event count (approx.): 0 lost: 0/0 drop: 0/0

The format is: <current period drop>/<total drop>

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-2lj87zz8tq9ye1ntax3ulw0n@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>