datapath: Tolerate backporting of rtnl_link_stats64 (as in RHEL 6).

datapath: Tolerate backporting of rtnl_link_stats64 (as in RHEL 6).

Red Hat Enterprise Linux 6 has a 2.6.32 kernel but it backports the
rtnl_link_stats64 structure that was introduced in 2.6.35, so we need to
check whether it was defined instead of just guessing based on the kernel
version number.

Build-tested only, on 2.6.32-71.14.1.el6 (RHEL 6),
linux-2.6.18-128.1.6.el5.xs5.5.0.496.101 (XenServer 5.5.0),
2.6.18-128.1.6.el5.xs5.5.0.505.1024xen (XenServer 5.5.0 update 1),
and upstream 2.6.18, 2.6.26, 2.6.29, 2.6.33, 2.6.34, 2.6.36, all for i386,
plus 2.6.36 for x86-64.

My machine's userspace headers have <linux/if_link.h> but not
rtnl_link_stats64.  Jesse Gross tested the case where <linux/if_link.h>
has rtnl_link_stats64, on Ubuntu 10.10.

Reported-by: Geoff White <gwhite@nicira.com>
Tested-by: Jesse Gross <jesse@nicira.com>