idp
fn
data
Iterate over the pointers registered with the given idr. The callback function will be called for each pointer currently registered, passing the id, the pointer and the data pointer passed to this function. It is not safe to modify the idr tree while in the callback, so functions such as idr_get_new and idr_remove are not allowed.
We check the return of fn each time. If it returns anything other than 0, we break out and return that value.
The caller must serialize idr_for_each vs idr_get_new and idr_remove.