sys: 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: NMiaohe Lin <linmiaohe@huawei.com>
Acked-by: NChristian Brauner <christian.brauner@ubuntu.com>
[christian.brauner@ubuntu.com: rewrite commit message]
Link: https://lore.kernel.org/r/20200814085718.40326-1-linmiaohe@huawei.comSigned-off-by: NChristian Brauner <christian.brauner@ubuntu.com>

kernel/sys.c

@@ -1753,7 +1753,7 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
 
 		if (who == RUSAGE_CHILDREN)
 			break;
-		/* fall through */
+		fallthrough;
 
 	case RUSAGE_SELF:
 		thread_group_cputime_adjusted(p, &tgutime, &tgstime);