Keep CARP state as INIT when net.inet.carp.allow=0.

Currently when net.inet.carp.allow=0 CARP state remains as MASTER, which is
not very useful (if there are other masters -- it can lead to split brain,
if there are none -- it makes no sense). Having it as INIT makes it clear
that carp packets are disabled.

Submitted by: wg
MFC after: 1 month
Relnotes: yes
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D14477