@post name is the NULL-terminated name of the next xattr, and val
contains the value of the xattr, which is of length len. If the end
of the list has been reached, name and val are NULL, and len is 0.
@param iter iterator to advance
@param name where to store the name of the next xattr
@param val where to store the value of the next xattr
@param len the number of bytes in val
@returns 0 on success, negative error code on failure
Get the next xattr on the object
@pre iter is a valid iterator
@post name is the NULL-terminated name of the next xattr, and val contains the value of the xattr, which is of length len. If the end of the list has been reached, name and val are NULL, and len is 0.
@param iter iterator to advance @param name where to store the name of the next xattr @param val where to store the value of the next xattr @param len the number of bytes in val @returns 0 on success, negative error code on failure