tun: zero the whole vnet header in tun_put_user()

In the Linux kernel, the following vulnerability has been resolved:

tun: zero the whole vnet header in tun_put_user()

tun_put_user() declares an on-stack struct virtio_net_hdr_v1_hash_tunnel

without zeroing it. For a non-tunnel skb, virtio_net_hdr_tnl_from_skb()

only initializes the first 10 bytes (sizeof(struct virtio_net_hdr)),

leaving bytes 10..23 (num_buffers and the hash/tunnel fields) as stack

garbage.

An unprivileged user can set the vnet header size to 24 with

TUNSETVNETHDRSZ, so __tun_vnet_hdr_put() copies all 24 bytes of the

partially-initialized struct to userspace, leaking 14 bytes of kernel

stack on every read of a non-tunnel packet.

Fix it the same way tun_get_user() already does by zeroing the whole

header right after declaration.