idr test suite: Create anchor before launching throbber

The throbber could race with creation of the anchor entry and cause the
IDR to have zero entries in it, which would cause the test to fail.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Author: Matthew Wilcox (Oracle)

tools/testing/radix-tree/idr-test.c

@@ -301,11 +301,11 @@ void idr_find_test_1(int anchor_id, int throbber_id)
 	pthread_t throbber;
 	time_t start = time(NULL);
 
-	pthread_create(&throbber, NULL, idr_throbber, &throbber_id);
-
 	BUG_ON(idr_alloc(&find_idr, xa_mk_value(anchor_id), anchor_id,
 				anchor_id + 1, GFP_KERNEL) != anchor_id);
 
+	pthread_create(&throbber, NULL, idr_throbber, &throbber_id);
+
 	rcu_read_lock();
 	do {
 		int id = 0;