pf_modevent() registers the generic netlink family at MOD_LOAD, before
vnet_pf_init() has set up the state hashes and locks the handlers use,
and pf_unload() unregisters it after vnet_pf_uninit() has torn them
down. Sending PFNL_CMD_GETSTATES while pf loads or unloads panics in
pf_handle_getstates() on a NULL or freed V_pf_idhash.

Register the family from a SYSINIT ordered after vnet_pf_init(), and
unregister it from the matching SYSUNINIT, before vnet_pf_uninit().

Fixes: 2cef62886dc7 ("pf: convert state retrieval to netlink")
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")