refscale: Allow refscale without RCU Tasks

Currently, a CONFIG_PREEMPT_NONE=y kernel substitutes normal RCU for
RCU Tasks.  Unless that kernel builds refscale, whether built-in or as a
module, in which case RCU Tasks is (unnecessarily) built in.  This both
increases kernel size and increases the complexity of certain tracing
operations.  This commit therefore decouples the presence of refscale
from the presence of RCU Tasks.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>