rcu: Move rcu_gp_slow() to ->gp_seq

This commit moves rcu_gp_slow() to ->gp_seq.  This function only uses
the grace-period number to modulate delay, so rcu_seq_ctr(rsp->gp_seq)
gets the same effect, at least in cases where the delay is to happen
more than four times per wrap of an unsigned long.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -1879,7 +1879,8 @@ static void note_gp_changes(struct rcu_state *rsp, struct rcu_data *rdp)
 static void rcu_gp_slow(struct rcu_state *rsp, int delay)
 {
 	if (delay > 0 &&
-	    !(rsp->gpnum % (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
+	    !(rcu_seq_ctr(rsp->gp_seq) %
+	      (rcu_num_nodes * PER_RCU_NODE_PERIOD * delay)))
 		schedule_timeout_uninterruptible(delay);
 }