perf/x86/ibs: Check syscall attribute flags

Current implementation simply ignores attribute flags. Thus, there is
no notification to userland of unsupported features. Check syscall's
attribute flags to let userland know if a feature is supported by the
kernel. This is also needed to distinguish between future kernels what
might support a feature.

Cc: <stable@vger.kernel.org> v3.5..
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20120910093018.GO8285@erda.amd.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/perf_event.h

@@ -274,6 +274,8 @@ struct perf_event_attr {
 	__u64	branch_sample_type; /* enum branch_sample_type */
 };
 
+#define perf_flags(attr)	(*(&(attr)->read_format + 1))
+
 /*
  * Ioctls that can be done on a perf event fd:
  */