tun: update file current position

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 782e38b..e26cbea 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1356,6 +1356,8 @@
 	ret = tun_do_read(tun, tfile, iocb, iv, len,
 			  file->f_flags & O_NONBLOCK);
 	ret = min_t(ssize_t, ret, len);
+	if (ret > 0)
+		iocb->ki_pos = ret;
 out:
 	tun_put(tun);
 	return ret;