rcu: Clarify role of ->expmaskinitnext

Analogy with the ->qsmaskinitnext field might lead one to believe that
->expmaskinitnext tracks online CPUs.  This belief is incorrect: Any CPU
that has ever been online will have its bit set in the ->expmaskinitnext
field.  This commit therefore adds a comment to make this clear, at
least to people who read comments.

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