net: nlmon: Simplify nlmon_get_stats64

Do not set rtnl_link_stats64 fields to zero, since they are zeroed
before ops->ndo_get_stats64 is called in core dev_get_stats() function.

Also, simplify the data collection by removing the temporary variable.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>