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/ipc/sem.c b/ipc/sem.c
index 8c0244e..f6c30a8 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -1691,7 +1691,7 @@ static long ksys_semctl(int semid, int semnum, int cmd, unsigned long arg, int v
 	case IPC_SET:
 		if (copy_semid_from_user(&semid64, p, version))
 			return -EFAULT;
-		/* fall through */
+		fallthrough;
 	case IPC_RMID:
 		return semctl_down(ns, semid, cmd, &semid64);
 	default:
@@ -1805,7 +1805,7 @@ static long compat_ksys_semctl(int semid, int semnum, int cmd, int arg, int vers
 	case IPC_SET:
 		if (copy_compat_semid_from_user(&semid64, p, version))
 			return -EFAULT;
-		/* fallthru */
+		fallthrough;
 	case IPC_RMID:
 		return semctl_down(ns, semid, cmd, &semid64);
 	default: