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: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -660,7 +660,7 @@ static ssize_t tun_chr_aio_write(struct kiocb *iocb, const struct iovec *iv,
 			      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)