which store the hook entry point locations for the various protocol
families and the hooks.
Using array results in compact c code when doing accesses, i.e.
x = rcu_dereference(net->nf.hooks[pf][hook]);
but its also wasting a lot of memory, as most families are
not used.
So split the array into those families that are used, which
are only 5 (instead of 13). In most cases, the 'pf' argument is
constant, i.e. gcc removes switch statement.