perf ordered_events: Add ordered_events__last_flush_time()

perf ordered_events: Add ordered_events__last_flush_time()

Allow callers to get the ordered_events last flush timestamp.

This is needed in perf inject to obey finished-round ordering when
injecting additional events (e.g. from a guest perf.data file) with
timestamps. Any additional events that have timestamps before the last
flush time must be injected before the corresponding FINISHED_ROUND event.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: kvm@vger.kernel.org
Link: https://lore.kernel.org/r/20220711093218.10967-4-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>