net/dns_resolver: use credential guards in dns_query()

Use credential guards for scoped credential override with automatic
restoration on scope exit.

Link: https://patch.msgid.link/[e-mail]
Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

net/dns_resolver/dns_query.c

···
 {
 	struct key *rkey;
 	struct user_key_payload *upayload;
-	const struct cred *saved_cred;
 	size_t typelen, desclen;
 	char *desc, *cp;
 	int ret, len;
···
 	/* make the upcall, using special credentials to prevent the use of
 	 * add_key() to preinstall malicious redirections
 	 */
-	saved_cred = override_creds(dns_resolver_cache);
-	rkey = request_key_net(&key_type_dns_resolver, desc, net, options);
-	revert_creds(saved_cred);
+	scoped_with_creds(dns_resolver_cache)
+		rkey = request_key_net(&key_type_dns_resolver, desc, net, options);
 	kfree(desc);
 	if (IS_ERR(rkey)) {
 		ret = PTR_ERR(rkey);