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>

arch/sh/kernel/disassemble.c

@@ -486,7 +486,7 @@ static void print_sh_insn(u32 memaddr, u16 insn)
 					pr_cont("xd%d", rn & ~1);
 					break;
 				}
-				/* else, fall through */
+				fallthrough;
 			case D_REG_N:
 				pr_cont("dr%d", rn);
 				break;
@@ -495,7 +495,7 @@ static void print_sh_insn(u32 memaddr, u16 insn)
 					pr_cont("xd%d", rm & ~1);
 					break;
 				}
-				/* else, fall through */
+				fallthrough;
 			case D_REG_M:
 				pr_cont("dr%d", rm);
 				break;

arch/sh/kernel/kgdb.c

@@ -266,7 +266,7 @@ int kgdb_arch_handle_exception(int e_vector, int signo, int err_code,
 		ptr = &remcomInBuffer[1];
 		if (kgdb_hex2long(&ptr, &addr))
 			linux_regs->pc = addr;
-		/* fallthrough */
+		fallthrough;
 	case 'D':
 	case 'k':
 		atomic_set(&kgdb_cpu_doing_single_step, -1);

arch/sh/kernel/signal_32.c

@@ -418,7 +418,7 @@ handle_syscall_restart(unsigned long save_r0, struct pt_regs *regs,
 		case -ERESTARTSYS:
 			if (!(sa->sa_flags & SA_RESTART))
 				goto no_system_call_restart;
-		/* fallthrough */
+			fallthrough;
 		case -ERESTARTNOINTR:
 			regs->regs[0] = save_r0;
 			regs->pc -= instruction_size(__raw_readw(regs->pc - 4));