rcu: use softirq instead of kthreads except when RCU_BOOST=y

This patch #ifdefs RCU kthreads out of the kernel unless RCU_BOOST=y,
thus eliminating context-switch overhead if RCU priority boosting has
not been configured.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>