pf: vnet-ify pf_hashsize, pf_hashmask, pf_srchashsize and V_pf_srchashmask

These variables are tunables, so in principle they never change at runtime.
That would mean they don't need to be tracked per-vnet.

However, they both can be decreased (back to their default values) if the
memory allocations for their respective tables fail, and these allocations are
per-vnet. That is, it's possible for a few vnets to be started and have the
tuned size for the hash and srchash tables only to have later vnets fail the
initial allocation and fall back to smaller allocations. That would confuse
the previously created vnets (because their actual table size and size/mask
variables would no longer match).

Avoid this by turning these into per-vnet variables.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 271f146955641857c93705b5b1916b8004e5623c)