perf record: Store PERF_RECORD_FINISHED_ROUND only for nonempty rounds

perf record: Store PERF_RECORD_FINISHED_ROUND only for nonempty rounds

Currently we store PERF_RECORD_FINISHED_ROUND event each time
we go throught mmap buffers no matter if it contains any data,
which is useless.

Forcing the PERF_RECORD_FINISHED_ROUND event to be stored any
time we finished the round AND wrote at least one event.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: David Ahern <dsahern@gmail.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1406300177-31805-19-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>