vlan: unnecessary to check if vlan_pcpu_stats is NULL

if allocating memory for vlan_pcpu_stats failed, the device can not be operated

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