dpif-netdev: Initialize upcall->packet when queuing to userspace.

Only the data and size members were being initialized, but all of them
should be.

This is inspired by commit b6f4590fa036 (dpif-netdev: Initialize
upcall->packet when queuing to userspace.) from master.  The background is
not exactly the same as on master (the commit that it references is not
on branch-2.3).

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>