macvlan: unify macvlan_pcpu_stats and vlan_pcpu_stats

They are same, so unify them as one; since macvlan is a kind of vlan,
vlan_pcpu_stats should be a proper name for vlan and macvlan.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>