rcu: grpnum just records group number

The ->grpnum field in the rcu_node structure contains the bit position
in this structure's parent's bitmasks, which is not the CPU number.
This commit therefore adjusts this field's comment accordingly.
Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.h

@@ -75,7 +75,7 @@ struct rcu_node {
 				/*  Only one bit will be set in this mask. */
 	int	grplo;		/* lowest-numbered CPU here. */
 	int	grphi;		/* highest-numbered CPU here. */
-	u8	grpnum;		/* CPU/group number for next level up. */
+	u8	grpnum;		/* group number for next level up. */
 	u8	level;		/* root is at level 0. */
 	bool	wait_blkd_tasks;/* Necessary to wait for blocked tasks to */
 				/*  exit RCU read-side critical sections */