net: convert net.passive from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index a24a575..31a2b51 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -5,6 +5,7 @@
 #define __NET_NET_NAMESPACE_H
 
 #include <linux/atomic.h>
+#include <linux/refcount.h>
 #include <linux/workqueue.h>
 #include <linux/list.h>
 #include <linux/sysctl.h>
@@ -46,7 +47,7 @@ struct netns_ipvs;
 #define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
 
 struct net {
-	atomic_t		passive;	/* To decided when the network
+	refcount_t		passive;	/* To decided when the network
 						 * namespace should be freed.
 						 */
 	atomic_t		count;		/* To decided when the network