udpv6: add the required annotation to mib type

In commit 029a3743 ("udp6: cleanup stats accounting in recvmsg()")
I forgot to add the percpu annotation for the mib pointer. Add it, and
make sparse happy.

Fixes: 029a3743

 ("udp6: cleanup stats accounting in recvmsg()")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>