rcu/nocb: Update comments to prepare for forward-progress work

This commit simply rewords comments to prepare for leader nocb kthreads
doing only grace-period work and callback shuffling.  This will mean
the addition of replacement kthreads to invoke callbacks.  The "leader"
and "follower" thus become less meaningful, so the commit changes no-CB
comments with these strings to "GP" and "CB", respectively.  (Give or
take the usual grammatical transformations.)

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