rcu: Implement lockdep_rcu_enabled for !CONFIG_DEBUG_LOCK_ALLOC

Provide an implementation for debug_lockdep_rcu_enabled() when
CONFIG_DEBUG_LOCK_ALLOC is not enabled. This allows code to check
if rcu lockdep debugging is available without needing an extra
check if CONFIG_DEBUG_LOCK_ALLOC is enabled.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>