rcu: Convert rcu_gpnum_ovf() to ->gp_seq

This commit converts rcu_gpnum_ovf() to use ->gp_seq instead of ->gpnum.
Same size unsigned long, so same approach.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -1089,14 +1089,15 @@ static int rcu_is_cpu_rrupt_from_idle(void)
 /*
  * We are reporting a quiescent state on behalf of some other CPU, so
  * it is our responsibility to check for and handle potential overflow
- * of the rcu_node ->gpnum counter with respect to the rcu_data counters.
+ * of the rcu_node ->gp_seq counter with respect to the rcu_data counters.
  * After all, the CPU might be in deep idle state, and thus executing no
  * code whatsoever.
  */
 static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
 {
 	raw_lockdep_assert_held_rcu_node(rnp);
-	if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4, rnp->gpnum))
+	if (ULONG_CMP_LT(rcu_seq_current(&rdp->gp_seq) + ULONG_MAX / 4,
+			 rnp->gp_seq))
 		WRITE_ONCE(rdp->gpwrap, true);
 	if (ULONG_CMP_LT(rdp->rcu_iw_gpnum + ULONG_MAX / 4, rnp->gpnum))
 		rdp->rcu_iw_gpnum = rnp->gpnum + ULONG_MAX / 4;