net, uapi: fix -Wpointer-arith warnings

Add casts to fix these warnings:

./usr/include/linux/netfilter_arp/arp_tables.h:200:19: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/netfilter_bridge/ebtables.h:197:19: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/netfilter_ipv4/ip_tables.h:223:19: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/netfilter_ipv6/ip6_tables.h:263:19: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/tipc_config.h:310:28: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/tipc_config.h:410:24: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]
./usr/include/linux/virtio_ring.h:170:16: error: pointer of type 'void *' used in arithmetic [-Werror=pointer-arith]

Those are theoretical probably but kernel doesn't control compiler flags...