tuntap: fix for "tuntap: XDP transmission"

Fixes: fc72d1d5 ("tuntap: XDP transmission")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>