rcu: Correct comment for values of ->gp_state field

This commit corrects the comment for the values of the ->gp_state field,
which previously incorrectly said that these were for the ->gp_flags
field.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcu/tree.h

@@ -529,7 +529,7 @@ struct rcu_state {
 #define RCU_GP_FLAG_INIT 0x1	/* Need grace-period initialization. */
 #define RCU_GP_FLAG_FQS  0x2	/* Need grace-period quiescent-state forcing. */
 
-/* Values for rcu_state structure's gp_flags field. */
+/* Values for rcu_state structure's gp_state field. */
 #define RCU_GP_IDLE	 0	/* Initial state and no GP in progress. */
 #define RCU_GP_WAIT_GPS  1	/* Wait for grace-period start. */
 #define RCU_GP_DONE_GPS  2	/* Wait done for grace-period start. */