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>

fs/fcntl.c

@@ -362,7 +362,7 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_OFD_SETLK:
 	case F_OFD_SETLKW:
 #endif
-		/* Fallthrough */
+		fallthrough;
 	case F_SETLK:
 	case F_SETLKW:
 		if (copy_from_user(&flock, argp, sizeof(flock)))
@@ -771,7 +771,7 @@ static void send_sigio_to_task(struct task_struct *p,
 			if (!do_send_sig_info(signum, &si, p, type))
 				break;
 		}
-		/* fall-through - fall back on the old plain SIGIO signal */
+			fallthrough;	/* fall back on the old plain SIGIO signal */
 		case 0:
 			do_send_sig_info(SIGIO, SEND_SIG_PRIV, p, type);
 	}