Force a statistics update when our ethtool gstats routine
is called.  Otherwise, ethtool will continue to read stale
stats until something forces an update by reading /proc/net/dev

This requires putting a lock around the stats update to guard
against 2 threads (one via ethtool, and one via procfs)
updating the stats at once.
Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: David S. Miller <davem@davemloft.net>