bonding: Use RCU_INIT_POINTER() for better overhead and for sparse

Although rcu_assign_pointer() can be used to assign a constant
NULL pointer, doing so gets you an unnecessary memory barrier and
in some circumstances, sparse warnings.  This commit therefore
changes the rcu_assign_pointer() of NULL in __bond_release_one() to
RCU_INIT_POINTER().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

drivers/net/bonding/bond_main.c

@@ -1763,7 +1763,7 @@ static int __bond_release_one(struct net_device *bond_dev,
 	}
 
 	if (all) {
-		rcu_assign_pointer(bond->curr_active_slave, NULL);
+		RCU_INIT_POINTER(bond->curr_active_slave, NULL);
 	} else if (oldcurrent == slave) {
 		/*
 		 * Note that we hold RTNL over this sequence, so there