treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
diff --git a/arch/x86/kernel/mpparse.c b/arch/x86/kernel/mpparse.c
index 411af4a..baa2109 100644
--- a/arch/x86/kernel/mpparse.c
+++ b/arch/x86/kernel/mpparse.c
@@ -312,7 +312,7 @@ static void __init construct_default_ioirq_mptable(int mpc_default_type)
 		case 2:
 			if (i == 0 || i == 13)
 				continue;	/* IRQ0 & IRQ13 not connected */
-			/* fall through */
+			fallthrough;
 		default:
 			if (i == 2)
 				continue;	/* IRQ2 is never connected */
@@ -356,7 +356,7 @@ static void __init construct_ioapic_table(int mpc_default_type)
 	default:
 		pr_err("???\nUnknown standard configuration %d\n",
 		       mpc_default_type);
-		/* fall through */
+		fallthrough;
 	case 1:
 	case 5:
 		memcpy(bus.bustype, "ISA   ", 6);