snmp: fix some left over of snmp stats

Fengguang reported the following sparse warning:

>> net/ipv6/proc.c:198:41: sparse: incorrect type in argument 1 (different address spaces)
   net/ipv6/proc.c:198:41:    expected void [noderef] <asn:3>*mib
   net/ipv6/proc.c:198:41:    got void [noderef] <asn:3>**pcpumib

Fixes: commit 698365fa

 (net: clean up snmp stats code)
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>