Fix per-jail zfs.mount_snapshot setting

When jail.conf set the nopersist flag during startup, it was
incorrectly destroying the per-jail ZFS settings.

PR: 260160
Reported by: imp (previous version), mm (upstream), freqlabs (upstream)
MFC after: immediately
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38662