signal: Convert to the new fallthrough notation

Switch from using the /* fall through */ comment style notation to the new,
preferred notation as outlined in our docs.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: rewrite commit message]
Link: https://lore.kernel.org/r/20200814083932.4975-1-linmiaohe@huawei.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

kernel/signal.c

@@ -851,7 +851,7 @@ static int check_kill_permission(int sig, struct kernel_siginfo *info,
 			 */
 			if (!sid || sid == task_session(current))
 				break;
-			/* fall through */
+			fallthrough;
 		default:
 			return -EPERM;
 		}