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

(cherry picked from commit 654c1b8ef3c0fcbad3a77ea2a6f35c0dffc86e88)