net: core: use this_cpu_ptr per-cpu helper

flush_tasklet is a struct, not a pointer in percpu var.
so use this_cpu_ptr to get the member pointer.

Signed-off-by: Shan Wei <davidshan@tencent.com>
Reviewed-by: Christoph Lameter <cl@linux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>