netdev: set __percpu attribute on netdev_alloc_pcpu_stats

This patch fixes sparse warnings in vlan driver.
It propagates the sparse __percpu attribute from alloc_percpu
into netdev_alloc_pcpu_stats. I expect it may trigger additional
sparse warnings from other drivers that are missing the __percpu
attribute.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Cong Wang <cwang@twopensource.com>
Signed-off-by: David S. Miller <davem@davemloft.net>