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>
diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 4dd5ee2..a0b97c4 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1763,7 +1763,7 @@
 	}
 
 	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