datapath: Always use generic stats for devices (vports)

datapath:  Always use generic stats for devices (vports)

    Currently ovs is using device stats for Linux devices and count them
itself in other situations. This leads to overlap with hardware stats,
inconsistencies, etc. It's much better to just always count the packets
flowing through the switch and let userspace do any merging that it wants.

Following patch removes vport->get_stats() interface. vport-stat is changed
to use new `struct ovs_vport_stat` rather than rtnl_link_stats64.
Definitions of rtnl_link_stats64 is removed from OVS.  dipf_port->stat is also
removed as aggregate stats are only available at netdev layer.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>