net: reintroduce missing rcu_assign_pointer() calls

commit a9b3cd7f32 (rcu: convert uses of rcu_assign_pointer(x, NULL) to
RCU_INIT_POINTER) did a lot of incorrect changes, since it did a
complete conversion of rcu_assign_pointer(x, y) to RCU_INIT_POINTER(x,
y).

We miss needed barriers, even on x86, when y is not NULL.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index e875f89..76613f5 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -777,7 +777,7 @@
 		if (exp->helper) {
 			help = nf_ct_helper_ext_add(ct, GFP_ATOMIC);
 			if (help)
-				RCU_INIT_POINTER(help->helper, exp->helper);
+				rcu_assign_pointer(help->helper, exp->helper);
 		}
 
 #ifdef CONFIG_NF_CONNTRACK_MARK