rcu: Make rcu_gp_cleanup() write only once to ->gp_flags

At the end of rcu_gp_cleanup(), if another grace period is needed, but
not via rcu_accelerate_cbs(), the ->gp_flags field is written twice,
once when making the new grace-period request, and once when clearing
all other types of requests.  This commit therefore adds an else-clause
to avoid this double write.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2117,8 +2117,9 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
 		rsp->gp_req_activity = jiffies;
 		trace_rcu_grace_period(rsp->name, READ_ONCE(rsp->gpnum),
 				       TPS("newreq"));
-	}
+	} else {
 		WRITE_ONCE(rsp->gp_flags, rsp->gp_flags & RCU_GP_FLAG_INIT);
+	}
 	raw_spin_unlock_irq_rcu_node(rnp);
 }