perf stat: Factor out body of event handling loop for fork case

Factor out body of event handling loop for fork case reusing
handle_interval() function.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/a8ae3f8d-a30e-fd40-998a-f5ca3e98cd45@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>