tuntap: enable bh early during processing XDP

This patch move the bh enabling a little bit earlier, this will be
used for factoring out the core XDP logic of tuntap.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>