Adding some prints for ordered events queue, to help debug issues.

Adding debug_ordered_events debug variable to be able to enable ordered
events debug messages using:

  $ perf --debug ordered-events=2 report ...

Also using oe pointer in perf_session__queue_event instead of chained
session variable dereferencing.
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-7p3mnnopjvsp9nmk9msqcfkm@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>