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.

Reported-by: Martin Matuska <mm@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Sponsored-by: Modirum MDPay
Sponsored-by: Klara, Inc.
Closes #14509