integrity: Annotate struct ima_rule_opt_list with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct ima_rule_opt_list.
Additionally, since the element count member must be set before accessing
the annotated flexible array member, move its initialization earlier.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Mimi Zohar <[e-mail]>
Cc: Dmitry Kasatkin <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: James Morris <[e-mail]>
Cc: "Serge E. Hallyn" <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Mimi Zohar <[e-mail]>
Reviewed-by: "Gustavo A. R. Silva" <[e-mail]>
Acked-by: Jarkko Sakkinen <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Kees Cook <[e-mail]>

security/integrity/ima/ima_policy.c

···
 
 struct ima_rule_opt_list {
 	size_t count;
-	char *items[];
+	char *items[] __counted_by(count);
 };
 
 /*
···
 		kfree(src_copy);
 		return ERR_PTR(-ENOMEM);
 	}
+	opt_list->count = count;
 
 	/*
 	 * strsep() has already replaced all instances of '|' with '\0',
···
 		opt_list->items[i] = cur;
 		cur = strchr(cur, '\0') + 1;
 	}
-	opt_list->count = count;
 
 	return opt_list;
 }