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>

kernel/rcu/rcutorture.c

@@ -1160,7 +1160,7 @@ rcutorture_extend_mask(int oldmask, struct torture_random_state *trsp)
 	unsigned long randmask2 = randmask1 >> 3;
 
 	WARN_ON_ONCE(mask >> RCUTORTURE_RDR_SHIFT);
-	/* Most of the time lots of bits, half the time only one bit. */
+	/* Mostly only one bit (need preemption!), sometimes lots of bits. */
 	if (!(randmask1 & 0x7))
 		mask = mask & randmask2;
 	else