pf: Increase default hash table size

Now that we (by default) limit the number of states to 100.000 it makse sense
to also adjust the default size of the hash table.

Based on the benchmarking results in
https://github.com/ocochard/netbenches/blob/master/Atom_C2758_8Cores-Chelsio_T540-CR/pf-states_hashsize/results/fbsd12-head.r332390/README.md
128K entries offers a good compromise between performance and memory use.

Users may still overrule this setting with the net.pf.states_hashsize and
net.pf.source_nodes_hashsize loader(8) tunables.