add clflush right after wmb in lbuf_xmit

- originally, pcie read request invalidates its address range carrying
No Snoop bit cleared in its TLP header. But, I found that not invalidating
packets are rarely happending, thereby breaking consistency between
SW and HW. We haven't found the cause by now, but clflush right before
write[q/l] solves the issue as a explicit cache flush from sw-side.