stats_lock is non useless, lets remove it.

Also, ndo_get_stats64() doesnt have to clear the stats, caller takes
care of this.

Note: folding 32bit fields in 64bit one is problematic when one of 32bit
values wraps : SNMP reader will see a ~2^32 back change in a 64bit
value. A future patch should fix this.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Stephen Hemminger <shemminger@vyatta.com>
CC: Andrew Gallatin <gallatin@myri.com>
CC: Brice Goglin <brice@myri.com>
Acked-by: Jon Mason <mason@myri.com>
Signed-off-by: David S. Miller <davem@davemloft.net>