tun: Fix merge error

When forward-porting the tun accounting patch I managed to break
the send path compltely by dropping the tun_get call.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 4825c52..a1b0697 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -660,7 +660,7 @@
 			      unsigned long count, loff_t pos)
 {
 	struct file *file = iocb->ki_filp;
-	struct tun_struct *tun = file->private_data;
+	struct tun_struct *tun = tun_get(file);
 	ssize_t result;
 
 	if (!tun)