ovl: port ovl_listxattr() to cred guard

Use the scoped ovl cred guard.

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

fs/overlayfs/xattrs.c

···
 	ssize_t res;
 	size_t len;
 	char *s;
-	const struct cred *old_cred;
 	size_t prefix_len, name_len;
 
-	old_cred = ovl_override_creds(dentry->d_sb);
-	res = vfs_listxattr(realdentry, list, size);
-	ovl_revert_creds(old_cred);
+	with_ovl_creds(dentry->d_sb)
+		res = vfs_listxattr(realdentry, list, size);
 	if (res <= 0 || size == 0)
 		return res;