tun: avoid calling xdp_rxq_info_unreg() twice

Similarly to tx ring, xdp_rxq_info is only registered
when !tfile->detached, so we need to avoid calling
xdp_rxq_info_unreg() twice too. The helper tun_cleanup_tx_ring()
already checks for this properly, so it is correct to put
xdp_rxq_info_unreg() just inside there.

Reported-by:  <syzbot+1c788d7ce0f0888f1d7f@syzkaller.appspotmail.com>
Fixes: 8565d26b ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net"

)
Cc: Jason Wang <jasowang@redhat.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>