rcu: Fix backwards RCU_LOCKDEP_WARN() in synchronize_rcu_tasks()

The RCU_LOCKDEP_WARN() in synchronize_rcu_tasks() triggers if the
scheduler is active, which is backwards.  This commit therefore
negates the test.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>