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
Reviewed by: imp (previous version), mm (upstream), freqlabs (upstream)
Approved by: re (cperciva)
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38662

(cherry picked from commit 8b04c1cbfc1cb71a1ce53b3a7855f1d45866fcfb)
(cherry picked from commit 426ed00525409d084e97dc44397722aff2cc0bb3)