tun: Adjust on-stack tun_page initialization.

Instead of constantly playing with the struct initializer
syntax trying to make gcc and CLang both happy, just clear
it out using memset().

>> drivers/net/tun.c:2503:42: warning: Using plain integer as NULL pointer
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -2500,10 +2500,12 @@ static int tun_sendmsg(struct socket *sock, struct msghdr *m, size_t total_len)
 		return -EBADFD;
 
 	if (ctl && (ctl->type == TUN_MSG_PTR)) {
-		struct tun_page tpage = {0};
+		struct tun_page tpage;
 		int n = ctl->num;
 		int flush = 0;
 
+		memset(&tpage, 0, sizeof(tpage));
+
 		local_bh_disable();
 		rcu_read_lock();