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 <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Baoquan He <[e-mail]>
Cc: Christophe Leroy <[e-mail]>
Cc: Daniel Thompson <[e-mail]>
Cc: David Wang <[e-mail]>
Cc: Douglas Anderson <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Cc: Kees Cook <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Mike Rapoport <[e-mail]>
Cc: Nicolai Stange <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Pu Wen <[e-mail]>
Cc: Suravee Suthikulpanit <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: x86-ml <[e-mail]>
Link: https://lkml.kernel.org/r/20190125183903.GA4712@embeddedor

arch/x86/kernel/apic/io_apic.c

···
 		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;
···
 		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

···
 	switch (leaf) {
 	case 1:
 		l1 = &l1i;
+		/* fall through */
 	case 0:
 		if (!l1->val)
 			return;

arch/x86/kernel/hw_breakpoint.c

···
 			hw->len = X86_BREAKPOINT_LEN_X;
 			return 0;
 		}
+		/* fall through */
 	default:
 		return -EINVAL;
 	}

arch/x86/kernel/kgdb.c

···
 		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

···
 		 * 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;