rcu: convert uses of rcu_assign_pointer(x, NULL) to RCU_INIT_POINTER

When assigning a NULL value to an RCU protected pointer, no barrier
is needed. The rcu_assign_pointer, used to handle that but will soon
change to not handle the special case.

Convert all rcu_assign_pointer of NULL value.

//smpl
@@ expression P; @@

- rcu_assign_pointer(P, NULL)
+ RCU_INIT_POINTER(P, NULL)

// </smpl>

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mac80211/iface.c

@@ -456,7 +456,7 @@ static void ieee80211_do_stop(struct ieee80211_sub_if_data *sdata,
 						 BSS_CHANGED_BEACON_ENABLED);
 
 		/* remove beacon */
-		rcu_assign_pointer(sdata->u.ap.beacon, NULL);
+		RCU_INIT_POINTER(sdata->u.ap.beacon, NULL);
 		synchronize_rcu();
 		kfree(old_beacon);