rcu: Eliminate a few CONFIG_RCU_NOCB_CPU_ALL #ifdefs

This commit converts several CONFIG_RCU_NOCB_CPU_ALL #ifdefs to
instead use IS_ENABLED().  This change should help avoid hiding
code from compiler diagnostics.

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

diff --cc include/linux/rcupdate.h
Simple merge

diff --cc include/linux/rcutree.h
Simple merge

diff --cc kernel/rcu/tree_plugin.h
Simple merge