perf/x86: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword as a replacement for
a /* fall through */ comment, instead of letting the code fall through
to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://github.com/KSPP/linux/issues/115

arch/x86/events/core.c

@@ -1174,7 +1174,7 @@ static inline void x86_assign_hw_event(struct perf_event *event,
 	case INTEL_PMC_IDX_METRIC_BASE ... INTEL_PMC_IDX_METRIC_END:
 		/* All the metric events are mapped onto the fixed counter 3. */
 		idx = INTEL_PMC_IDX_FIXED_SLOTS;
-		/* fall through */
+		fallthrough;
 	case INTEL_PMC_IDX_FIXED ... INTEL_PMC_IDX_FIXED_BTS-1:
 		hwc->config_base = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
 		hwc->event_base = MSR_ARCH_PERFMON_FIXED_CTR0 +