tun: don't zeroize sock->file on detach

This is a fix for bug, introduced in 3.4 kernel by commit
1ab5ecb90cb6a3df1476e052f76a6e8f6511cb3d ("tun: don't hold network
namespace by tun sockets"), which, among other things, replaced simple
sock_put() by sk_release_kernel(). Below is sequence, which leads to
oops for non-persistent devices:

tun_chr_close()
tun_detach()				<== tun->socket.file = NULL
tun_free_netdev()
sk_release_sock()
sock_release(sock->file == NULL)
iput(SOCK_INODE(sock))			<== dereference on NULL pointer

This patch just removes zeroing of socket's file from __tun_detach().
sock_release() will do this.

Cc: stable@vger.kernel.org
Reported-by: Ruan Zhijie <ruanzhijie@hotmail.com>
Tested-by: Ruan Zhijie <ruanzhijie@hotmail.com>
Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -187,7 +187,6 @@ static void __tun_detach(struct tun_struct *tun)
 	netif_tx_lock_bh(tun->dev);
 	netif_carrier_off(tun->dev);
 	tun->tfile = NULL;
-	tun->socket.file = NULL;
 	netif_tx_unlock_bh(tun->dev);
 
 	/* Drop read queue */