net: align pcpu_sw_netstats and pcpu_lstats structs

Do not risk spanning these small structures on two cache lines,
it is absolutely not worth it.

For 32bit arches, the hint might not be enough, but we do not
really care anymore.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>