rcu: Eliminate BUG_ON() for kernel/rcu/tree.c

The tree.c file has a number of calls to BUG_ON(), which panics the
kernel, which is not a good strategy for devices (like embedded) that
don't have a way to capture console output. This commit therefore
converts these BUG_ON() calls to WARN_ON_ONCE() and WARN_ONCE().

Reported-by: Linus Torvalds <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

kernel/rcu/tree.c

···
 		 * Very early boot, before rcu_init().  Initialize if needed
 		 * and then drop through to queue the callback.
 		 */
-		BUG_ON(cpu != -1);
+		WARN_ON_ONCE(cpu != -1);
 		WARN_ON_ONCE(!rcu_is_watching());
 		if (rcu_segcblist_empty(&rdp->cblist))
 			rcu_segcblist_init(&rdp->cblist);
···
 
 	rcu_scheduler_fully_active = 1;
 	t = kthread_create(rcu_gp_kthread, NULL, "%s", rcu_state.name);
-	BUG_ON(IS_ERR(t));
+	if (WARN_ONCE(IS_ERR(t), "%s: Could not start grace-period kthread, OOM is now expected behavior\n", __func__))
+		return 0;
 	rnp = rcu_get_root();
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rcu_state.gp_kthread = t;