sev-dev: use prepare credential guard

Use the prepare credential guard for allocating a new set of
credentials.

Link: https://patch.msgid.link/20251103-work-creds-guards-prepare_creds-v1-3-b447b82f2c9b@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

drivers/crypto/ccp/sev-dev.c

@@ -261,22 +261,22 @@ static struct file *open_file_as_root(const char *filename, int flags, umode_t m
 {
 	struct path root __free(path_put) = {};
 	struct file *fp;
-	struct cred *cred;
 	const struct cred *old_cred;
 
 	task_lock(&init_task);
 	get_fs_root(init_task.fs, &root);
 	task_unlock(&init_task);
 
-	cred = prepare_creds();
+	CLASS(prepare_creds, cred)();
 	if (!cred)
 		return ERR_PTR(-ENOMEM);
+
 	cred->fsuid = GLOBAL_ROOT_UID;
 	old_cred = override_creds(cred);
 
 	fp = file_open_root(&root, filename, flags, mode);
 
-	put_cred(revert_creds(old_cred));
+	revert_creds(old_cred);
 
 	return fp;
 }