net, ipv6: convert inet6_dev.refcnt 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>

include/net/if_inet6.h

@@ -17,6 +17,7 @@
 
 #include <net/snmp.h>
 #include <linux/ipv6.h>
+#include <linux/refcount.h>
 
 /* inet6_dev.if_flags */
 
@@ -187,7 +188,7 @@ struct inet6_dev {
 
 	struct ifacaddr6	*ac_list;
 	rwlock_t		lock;
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 	__u32			if_flags;
 	int			dead;