perf/x86/amd/ibs: Fix pmu::stop() nesting

Patch 5a50f529 ("perf/x86/ibs: Fix race with IBS_STARTING state")
closed a big hole while opening another, smaller hole.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: 5a50f529 ("perf/x86/ibs: Fix race with IBS_STARTING state")
Signed-off-by: Ingo Molnar <mingo@kernel.org>