pfsync: Add CTLFLAG_VNET to carp_demotion_factor sysctl

pfsync: Add CTLFLAG_VNET to carp_demotion_factor sysctl

When trying to avoid a CARP demotion during a pfsync service restart, I
noticed that a non-default value for the net.pfsync.carp_demotion_factor
sysctl was not being applied during the demotion. The CARP was always
demoted by 240.

After investigating, I realized that the sysctl was using VNET_NAME()
without the CTLFLAG_VNET.

PR:		262983
Reviewed by:	kp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D34737