perf events: Precalculate the header space for PERF_SAMPLE_ fields

PERF_SAMPLE_{CALLCHAIN,RAW} have variable lenghts per sample, but the others
can be precalculated, reducing a bit the per sample cost.

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Ian Munsie <imunsie@au1.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

include/linux/perf_event.h

@@ -758,6 +758,8 @@ struct perf_event {
 	u64				shadow_ctx_time;
 
 	struct perf_event_attr		attr;
+	u16				header_size;
+	u16				read_size;
 	struct hw_perf_event		hw;
 
 	struct perf_event_context	*ctx;