datapath: Use "struct rtnl_link_stats64" instead of "struct odp_vport_stats".

Linux 2.6.35 added struct rtnl_link_stats64, which as a set of 64-bit
network device counters is what the OVS datapath needs.  We might as well
use it instead of our own.

This commit moves the if_link.h compat header from datapath/ into the
top-level include/ directory so that it is visible both to kernel and
userspace code.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>