dpif: Accept 64 kB packets from the kernel.

dpif: Accept 64 kB packets from the kernel.

Under virtualization (e.g. Xen) we sometimes receive packets as large as
64 kB, so the dpif code has to be prepared to accept it.

The only place where we keep these packets around for any amount of time
is when we need to schedule them for sending later, so trim them down
from 64 kB to their actual size in that case.