idr: Warn if old iterators see large IDs

Now that the IDR can be used to store large IDs, it is possible somebody
might only partially convert their old code and use the iterators which
can only handle IDs up to INT_MAX.  It's probably unwise to show them a
truncated ID, so settle for spewing warnings to dmesg, and terminating
the iteration.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>

lib/idr.c

@@ -159,7 +159,11 @@ int idr_for_each(const struct idr *idr,
 	void __rcu **slot;
 
 	radix_tree_for_each_slot(slot, &idr->idr_rt, &iter, 0) {
-		int ret = fn(iter.index, rcu_dereference_raw(*slot), data);
+		int ret;
+
+		if (WARN_ON_ONCE(iter.index > INT_MAX))
+			break;
+		ret = fn(iter.index, rcu_dereference_raw(*slot), data);
 		if (ret)
 			return ret;
 	}
@@ -187,6 +191,9 @@ void *idr_get_next(struct idr *idr, int *nextid)
 	if (!slot)
 		return NULL;
 
+	if (WARN_ON_ONCE(iter.index > INT_MAX))
+		return NULL;
+
 	*nextid = iter.index;
 	return rcu_dereference_raw(*slot);
 }