x86/kernel: Mark expected switch-case fall-throughs

In preparation to enable -Wimplicit-fallthrough by default, mark
switch-case statements where fall-through is intentional, explicitly in
order to fix a couple of -Wimplicit-fallthrough warnings.

Warning level 3 was used: -Wimplicit-fallthrough=3.

 [ bp: Massasge and trim commit message. ]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: David Wang <davidwang@zhaoxin.com>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Beulich <JBeulich@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nicolai Stange <nstange@suse.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Pu Wen <puwen@hygon.cn>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190125183903.GA4712@embeddedor

arch/x86/kernel/apic/io_apic.c

@@ -812,6 +812,7 @@ static int irq_polarity(int idx)
 		return IOAPIC_POL_HIGH;
 	case MP_IRQPOL_RESERVED:
 		pr_warn("IOAPIC: Invalid polarity: 2, defaulting to low\n");
+		/* fall through */
 	case MP_IRQPOL_ACTIVE_LOW:
 	default: /* Pointless default required due to do gcc stupidity */
 		return IOAPIC_POL_LOW;
@@ -859,6 +860,7 @@ static int irq_trigger(int idx)
 		return IOAPIC_EDGE;
 	case MP_IRQTRIG_RESERVED:
 		pr_warn("IOAPIC: Invalid trigger mode 2 defaulting to level\n");
+		/* fall through */
 	case MP_IRQTRIG_LEVEL:
 	default: /* Pointless default required due to do gcc stupidity */
 		return IOAPIC_LEVEL;

arch/x86/kernel/cpu/cacheinfo.c

@@ -248,6 +248,7 @@ amd_cpuid4(int leaf, union _cpuid4_leaf_eax *eax,
 	switch (leaf) {
 	case 1:
 		l1 = &l1i;
+		/* fall through */
 	case 0:
 		if (!l1->val)
 			return;

arch/x86/kernel/hw_breakpoint.c

@@ -279,6 +279,7 @@ static int arch_build_bp_info(struct perf_event *bp,
 			hw->len = X86_BREAKPOINT_LEN_X;
 			return 0;
 		}
+		/* fall through */
 	default:
 		return -EINVAL;
 	}

arch/x86/kernel/kgdb.c

@@ -467,6 +467,7 @@ int kgdb_arch_handle_exception(int e_vector, int signo, int err_code,
 		ptr = &remcomInBuffer[1];
 		if (kgdb_hex2long(&ptr, &addr))
 			linux_regs->ip = addr;
+		/* fall through */
 	case 'D':
 	case 'k':
 		/* clear the trace bit */

arch/x86/kernel/uprobes.c

@@ -745,6 +745,7 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 		 * OPCODE1() of the "short" jmp which checks the same condition.
 		 */
 		opc1 = OPCODE2(insn) - 0x10;
+		/* fall through */
 	default:
 		if (!is_cond_jmp_opcode(opc1))
 			return -ENOSYS;