netfilter: Fix format string of nfnetlink_queue proc file

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

Fixes:
$ cat /proc/net/netfilter/nfnetlink_queue
    0  29508   278 2 65531     0 2004213241 -2129885586  1
    1 -27747     0 2 65531     0     0        0  1
    2 -27748     0 2 65531     0     0        0  1

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>