netfilter: fix various sparse warnings

netfilter: fix various sparse warnings

net/bridge/br_netfilter.c:870:6: symbol 'br_netfilter_enable' was not declared. Should it be static?
  no; add include
net/ipv4/netfilter/nft_reject_ipv4.c:22:6: symbol 'nft_reject_ipv4_eval' was not declared. Should it be static?
  yes
net/ipv6/netfilter/nf_reject_ipv6.c:16:6: symbol 'nf_send_reset6' was not declared. Should it be static?
  no; add include
net/ipv6/netfilter/nft_reject_ipv6.c:22:6: symbol 'nft_reject_ipv6_eval' was not declared. Should it be static?
  yes
net/netfilter/core.c:33:32: symbol 'nf_ipv6_ops' was not declared. Should it be static?
  no; add include
net/netfilter/xt_DSCP.c:40:57: cast truncates bits from constant value (ffffff03 becomes 3)
net/netfilter/xt_DSCP.c:57:59: cast truncates bits from constant value (ffffff03 becomes 3)
  add __force, 3 is what we want.
net/ipv4/netfilter/nf_log_arp.c:77:6: symbol 'nf_log_arp_packet' was not declared. Should it be static?
  yes
net/ipv4/netfilter/nf_reject_ipv4.c:17:6: symbol 'nf_send_reset' was not declared. Should it be static?
  no; add include

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>