perf tools: Allow out of order messages in forced flush

In forced flush (OE_FLUSH__HALF) we break the rules of the flush
timestamp via PERF_RECORD_FINISHED_ROUND event, so we could get out of
order event.

Do not force error in this case plus changing the output warning to use
WARN_ONCE.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
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/n/tip-8q8794a8nlmpd1u8xrqmcyd2@git.kernel.org


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