drbd: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The use of "rcu_assign_pointer()" is NULLing out the pointer. According to RCU_INIT_POINTER()'s block comment:
"1. This use of RCU_INIT_POINTER() is NULLing out the pointer"
it is better to use it instead of rcu_assign_pointer() because it has a smaller overhead.

The following Coccinelle semantic patch was used:
@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., NULL)

---
 drivers/block/drbd/drbd_state.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index a5d8aae..73bc23f 100644
@@ -1563,7 +1563,7 @@ static int w_after_conn_state_ch(struct drbd_work *w, int unused)
 		old_conf = connection->net_conf;
 		connection->my_addr_len = 0;
 		connection->peer_addr_len = 0;
-		rcu_assign_pointer(connection->net_conf, NULL);
+		RCU_INIT_POINTER(connection->net_conf, NULL);
 		conn_free_crypto(connection);
 		mutex_unlock(&connection->resource->conf_update);
 

1.9.1