netfilter: Fix format string of nfnetlink_log proc file

The printed values are all of type unsigned integer, therefore use
%u instead of %d. Otherwise an user can face negative values.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>