perf: Optimize group_sched_in()

Use the ctx pmu instead of the event pmu.

When a group leader is a software event but the group contains
hardware events, the entire group is on the hardware PMU.

Using the hardware PMU for the transaction makes most sense since
that's the most expensive one to programm (and software PMUs generally
don't have TXN support anyway).
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-sctoo9t2f3nn2c9g568928q3@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -1733,7 +1733,7 @@ group_sched_in(struct perf_event *group_event,
 	       struct perf_event_context *ctx)
 {
 	struct perf_event *event, *partial_group = NULL;
-	struct pmu *pmu = group_event->pmu;
+	struct pmu *pmu = ctx->pmu;
 	u64 now = ctx->time;
 	bool simulate = false;