rcu: Convert ->need_future_gp[] array to boolean

There is no longer any need for ->need_future_gp[] to count the number of
requests for future grace periods, so this commit converts the additions
to assignments to "true" and reduces the size of each element to one byte.
While we are in the area, fix an obsolete comment.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>