netfilter: reduce size of hook entry points

struct net contains:

struct nf_hook_entries __rcu *hooks[NFPROTO_NUMPROTO][NF_MAX_HOOKS];

where NFPROTO_NUMPROTO = 13 and NF_MAX_HOOKS = 8.

... and that needs a *lot* more space than what we really need. We only need hooks for arp, bridge, ipv4, ipv6 and decnet.

Arp only has 3 hook types, decnet has 7, all others have 5. So replace this with dedicated arrays of the correct size to save some space.

Changes since RFC:
- bridge only needs 5, not 6 hooks (BROUTE isn't a real hookpoint)
- Use run-time check to reject register requests for hook types that don't fit the array size.
RFC tried to use BUILD_BUG_ON checks in nf_hook(), but that breaks build on some older gcc releases.

 include/linux/netfilter.h       |   30 ++++++++++++++++++--
 include/net/netns/netfilter.h   |   15 +++++++++-
 net/bridge/br_netfilter_hooks.c |    2 -
 net/netfilter/core.c            |   60 ++++++++++++++++++++++++++++++++++------
 net/netfilter/nf_queue.c        |   21 ++++++++++++--
 5 files changed, 114 insertions(+), 14 deletions(-)