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

Commit db73f7166a6 (netdev-dpdk: Fix race condition with DPDK mempools in
non pmd threads) switched to a new way of setting up 'upcall->packet', but
only initialized two of the fields in the packet.  This could cause
core dumps and other strange behavior.  In particular it caused failures in
several unit tests on XenServer.

This commit fixes the problem by initializing the entire ofpbuf.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>