Revert "tuntap: add missing xdp flush"

This reverts commit 762c330d. The
reason is we try to batch packets for devmap which causes calling
xdp_do_flush() in the process context. Simply disabling preemption
may not work since process may move among processors which lead
xdp_do_flush() to miss some flushes on some processors.

So simply revert the patch, a follow-up patch will add the xdp flush
correctly.

Reported-by: Christoffer Dall <christoffer.dall@linaro.org>
Fixes: 762c330d ("tuntap: add missing xdp flush")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>