ARM/hw_breakpoint: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

arch/arm/kernel/hw_breakpoint.c: In function 'hw_breakpoint_arch_parse':
arch/arm/kernel/hw_breakpoint.c:609:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
      ^
arch/arm/kernel/hw_breakpoint.c:611:2: note: here
  case 3:
  ^~~~
arch/arm/kernel/hw_breakpoint.c:613:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
      ^
arch/arm/kernel/hw_breakpoint.c:615:2: note: here
  default:
  ^~~~~~~
arch/arm/kernel/hw_breakpoint.c: In function 'arch_build_bp_info':
arch/arm/kernel/hw_breakpoint.c:544:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if ((hw->ctrl.type != ARM_BREAKPOINT_EXECUTE)
      ^
arch/arm/kernel/hw_breakpoint.c:547:2: note: here
  default:
  ^~~~~~~
In file included from include/linux/kernel.h:11,
                 from include/linux/list.h:9,
                 from include/linux/preempt.h:11,
                 from include/linux/hardirq.h:5,
                 from arch/arm/kernel/hw_breakpoint.c:16:
arch/arm/kernel/hw_breakpoint.c: In function 'hw_breakpoint_pending':
include/linux/compiler.h:78:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
 # define unlikely(x) __builtin_expect(!!(x), 0)
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
include/asm-generic/bug.h:136:2: note: in expansion of macro 'unlikely'
  unlikely(__ret_warn_on);     \
  ^~~~~~~~
arch/arm/kernel/hw_breakpoint.c:863:3: note: in expansion of macro 'WARN'
   WARN(1, "Asynchronous watchpoint exception taken. Debugging results may be unreliable\n");
   ^~~~
arch/arm/kernel/hw_breakpoint.c:864:2: note: here
  case ARM_ENTRY_SYNC_WATCHPOINT:
  ^~~~
arch/arm/kernel/hw_breakpoint.c: In function 'core_has_os_save_restore':
arch/arm/kernel/hw_breakpoint.c:910:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (oslsr & ARM_OSLSR_OSLM0)
      ^
arch/arm/kernel/hw_breakpoint.c:912:2: note: here
  default:
  ^~~~~~~
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

arch/arm/kernel/hw_breakpoint.c

@@ -544,6 +544,7 @@ static int arch_build_bp_info(struct perf_event *bp,
 		if ((hw->ctrl.type != ARM_BREAKPOINT_EXECUTE)
 			&& max_watchpoint_len >= 8)
 			break;
+		/* Else, fall through */
 	default:
 		return -EINVAL;
 	}
@@ -608,10 +609,12 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
 		/* Allow halfword watchpoints and breakpoints. */
 		if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
 			break;
+		/* Else, fall through */
 	case 3:
 		/* Allow single byte watchpoint. */
 		if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
 			break;
+		/* Else, fall through */
 	default:
 		ret = -EINVAL;
 		goto out;
@@ -861,6 +864,7 @@ static int hw_breakpoint_pending(unsigned long addr, unsigned int fsr,
 		break;
 	case ARM_ENTRY_ASYNC_WATCHPOINT:
 		WARN(1, "Asynchronous watchpoint exception taken. Debugging results may be unreliable\n");
+		/* Fall through */
 	case ARM_ENTRY_SYNC_WATCHPOINT:
 		watchpoint_handler(addr, fsr, regs);
 		break;
@@ -909,6 +913,7 @@ static bool core_has_os_save_restore(void)
 		ARM_DBG_READ(c1, c1, 4, oslsr);
 		if (oslsr & ARM_OSLSR_OSLM0)
 			return true;
+		/* Else, fall through */
 	default:
 		return false;
 	}