pf: fix pfsync breaking carp

Fix missing initialisation of sc_flags into a valid sync state on clone which
breaks carp in pfsync.

This regression was introduce by r342051.

PR: 235005
Submitted by: smh@FreeBSD.org
Pointy hat to: kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D18882