idr test suite: Take RCU read lock in idr_find_test_1

When run on a single CPU, this test would frequently access already-freed
memory.  Due to timing, this bug never showed up on multi-CPU tests.

Reported-by: Chris von Recklinghausen <crecklin@redhat.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

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

@@ -306,11 +306,15 @@ void idr_find_test_1(int anchor_id, int throbber_id)
 	BUG_ON(idr_alloc(&find_idr, xa_mk_value(anchor_id), anchor_id,
 				anchor_id + 1, GFP_KERNEL) != anchor_id);
 
+	rcu_read_lock();
 	do {
 		int id = 0;
 		void *entry = idr_get_next(&find_idr, &id);
+		rcu_read_unlock();
 		BUG_ON(entry != xa_mk_value(id));
+		rcu_read_lock();
 	} while (time(NULL) < start + 11);
+	rcu_read_unlock();
 
 	pthread_join(throbber, NULL);