MIPS: perf events: handle switch statement falling through warnings

Now that we build with -Wimplicit-fallthrough=3, some warnings are
produced in the arch/mips perf events code that are promoted to errors:

 arch/mips/kernel/perf_event_mipsxx.c:792:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/kernel/perf_event_mipsxx.c:795:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/kernel/perf_event_mipsxx.c:798:3: error: this statement may fall through [-Werror=implicit-fallthrough=]
 arch/mips/kernel/perf_event_mipsxx.c:1407:6: error: this statement may fall through [-Werror=implicit-fallthrough=]

Assume the fall throughs are deliberate amd annotate/eliminate them.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[paul.burton@mips.com:
  - Make n signed to fix the loop condition.
  - Simplify the initialization of n, which should never have a value
    greater than 4.
  - Invert conditions in the loop to decrease indentation.]
Signed-off-by: Paul Burton <paul.burton@mips.com>

arch/mips/kernel/perf_event_mipsxx.c

@@ -793,15 +793,19 @@ static void reset_counters(void *arg)
 	case 4:
 		mipsxx_pmu_write_control(3, 0);
 		mipspmu.write_counter(3, 0);
+		/* fall through */
 	case 3:
 		mipsxx_pmu_write_control(2, 0);
 		mipspmu.write_counter(2, 0);
+		/* fall through */
 	case 2:
 		mipsxx_pmu_write_control(1, 0);
 		mipspmu.write_counter(1, 0);
+		/* fall through */
 	case 1:
 		mipsxx_pmu_write_control(0, 0);
 		mipspmu.write_counter(0, 0);
+		/* fall through */
 	}
 }
 
@@ -1383,7 +1387,7 @@ static int mipsxx_pmu_handle_shared_irq(void)
 	struct perf_sample_data data;
 	unsigned int counters = mipspmu.num_counters;
 	u64 counter;
-	int handled = IRQ_NONE;
+	int n, handled = IRQ_NONE;
 	struct pt_regs *regs;
 
 	if (cpu_has_perf_cntr_intr_bit && !(read_c0_cause() & CAUSEF_PCI))
@@ -1404,20 +1408,16 @@ static int mipsxx_pmu_handle_shared_irq(void)
 
 	perf_sample_data_init(&data, 0, 0);
 
-	switch (counters) {
-#define HANDLE_COUNTER(n)						\
-	case n + 1:							\
-		if (test_bit(n, cpuc->used_mask)) {			\
-			counter = mipspmu.read_counter(n);		\
-			if (counter & mipspmu.overflow) {		\
-				handle_associated_event(cpuc, n, &data, regs); \
-				handled = IRQ_HANDLED;			\
-			}						\
-		}
-	HANDLE_COUNTER(3)
-	HANDLE_COUNTER(2)
-	HANDLE_COUNTER(1)
-	HANDLE_COUNTER(0)
+	for (n = counters - 1; n >= 0; n--) {
+		if (!test_bit(n, cpuc->used_mask))
+			continue;
+
+		counter = mipspmu.read_counter(n);
+		if (!(counter & mipspmu.overflow))
+			continue;
+
+		handle_associated_event(cpuc, n, &data, regs);
+		handled = IRQ_HANDLED;
 	}
 
 #ifdef CONFIG_MIPS_PERF_SHARED_TC_COUNTERS