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

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: [email protected]
Reported-by: Ruan Zhijie <[email protected]>
Tested-by: Ruan Zhijie <[email protected]>
Acked-by: Al Viro <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Acked-by: Yuchung Cheng <[email protected]>
Signed-off-by: Stanislav Kinsbursky <[email protected]>
Signed-off-by: David S. Miller <[email protected]>