tuntap: correctly add the missing XDP flush

tuntap: correctly add the missing XDP flush

We don't flush batched XDP packets through xdp_do_flush_map(), this
will cause packets stall at TX queue. Consider we don't do XDP on NAPI
poll(), the only possible fix is to call xdp_do_flush_map()
immediately after xdp_do_redirect().

Note, this in fact won't try to batch packets through devmap, we could
address in the future.

Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
Fixes: 761876c857cb ("tap: XDP support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>