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>

kernel/kallsyms.c

@@ -684,12 +684,12 @@ bool kallsyms_show_value(const struct cred *cred)
 	case 0:
 		if (kallsyms_for_perf())
 			return true;
-	/* fallthrough */
+		fallthrough;
 	case 1:
 		if (security_capable(cred, &init_user_ns, CAP_SYSLOG,
 				     CAP_OPT_NOAUDIT) == 0)
 			return true;
-	/* fallthrough */
+		fallthrough;
 	default:
 		return false;
 	}