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/csky/kernel/signal.c

@@ -194,7 +194,7 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 				regs->a0 = -EINTR;
 				break;
 			}
-			/* fallthrough */
+			fallthrough;
 		case -ERESTARTNOINTR:
 			regs->a0 = regs->orig_a0;
 			regs->pc -= TRAP0_SIZE;