Fix per-jail zfs.mount_snapshot setting

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)