gen_stats.c: Add description for cpu argument

Function gnet_stats_copy_basic is missing the description of the cpu
argument in the documentation. Adding it.
Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/gen_stats.c

@@ -140,6 +140,7 @@ EXPORT_SYMBOL(__gnet_stats_copy_basic);
 /**
  * gnet_stats_copy_basic - copy basic statistics into statistic TLV
  * @d: dumping handle
+ * @cpu: copy statistic per cpu
  * @b: basic statistics
  *
  * Appends the basic statistics to the top level TLV created by