pfsync: Fix state sync during initial bulk update

States learned via pfsync from a peer with the same ruleset checksum were not
getting assigned to rules like they should because pfsync_in_upd() wasn't
passing the PFSYNC_SI_CKSUM flag along to pfsync_state_import.

PR: 229092
Submitted by: Kajetan Staszkiewicz <vegeta tuxpowered.net>
Obtained from: OpenBSD
MFC after: 1 week
Sponsored by: InnoGames GmbH