rcu: Fix rcu_{node,data} comments about gp_seq_needed

Recent changes have removed the old ->gp_seq_needed field from the
rcu_state structure, which in turn obsoleted a couple of comments in
the rcu_node and rcu_data structures.  This commit therefore updates
these comments accordingly.
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: <kernel-team@android.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree.h

@@ -57,7 +57,7 @@ struct rcu_node {
 					/*  some rcu_state fields as well as */
 					/*  following. */
 	unsigned long gp_seq;	/* Track rsp->rcu_gp_seq. */
-	unsigned long gp_seq_needed; /* Track rsp->rcu_gp_seq_needed. */
+	unsigned long gp_seq_needed; /* Track furthest future GP request. */
 	unsigned long completedqs; /* All QSes done for this node. */
 	unsigned long qsmask;	/* CPUs or groups that need to switch in */
 				/*  order for current grace period to proceed.*/
@@ -163,7 +163,7 @@ union rcu_noqs {
 struct rcu_data {
 	/* 1) quiescent-state and grace-period handling : */
 	unsigned long	gp_seq;		/* Track rsp->rcu_gp_seq counter. */
-	unsigned long	gp_seq_needed;	/* Track rsp->rcu_gp_seq_needed ctr. */
+	unsigned long	gp_seq_needed;	/* Track furthest future GP request. */
 	union rcu_noqs	cpu_no_qs;	/* No QSes yet for this CPU. */
 	bool		core_needs_qs;	/* Core waits for quiesc state. */
 	bool		beenonline;	/* CPU online at least once. */