tun: Fix GSO meta-data handling in tun_get_user

When we write the GSO meta-data in tun_get_user we end up advancing
the IO vector twice, thus exhausting the user buffer before we can
finish writing the packet.

Fixes: f5ff53b4

 ("{macvtap,tun}_get_user(): switch to iov_iter")
Reported-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>