rcutorture: Make rcutorture_extend_mask() comment match the code

The code actually rarely uses more than one type of RCU read-side
protection, as is actually desired given that we need some reasonable
probability of preempting RCU read-side critical sections, which cannot
happen with multiple types of protection.  This comment therefore adjusts
the comment.

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