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 openzfs#14509

module/os/freebsd/zfs/zfs_vfsops.c

@@ -2495,7 +2495,9 @@ zfs_jailparam_set(void *obj, void *data)
 		mount_snapshot = -1;
 	else
 		jsys = JAIL_SYS_NEW;
-	if (jsys == JAIL_SYS_NEW) {
+	switch (jsys) {
+	case JAIL_SYS_NEW:
+	{
 		/* "zfs=new" or "zfs.*": the prison gets its own ZFS info. */
 		struct zfs_jailparam *zjp;
 
@@ -2513,12 +2515,22 @@ zfs_jailparam_set(void *obj, void *data)
 		if (mount_snapshot != -1)
 			zjp->mount_snapshot = mount_snapshot;
 		mtx_unlock(&pr->pr_mtx);
-	} else {
+		break;
+	}
+	case JAIL_SYS_INHERIT:
 		/* "zfs=inherit": inherit the parent's ZFS info. */
 		mtx_lock(&pr->pr_mtx);
 		osd_jail_del(pr, zfs_jailparam_slot);
 		mtx_unlock(&pr->pr_mtx);
+		break;
+	case -1:
+		/*
+		 * If the setting being changed is not ZFS related
+		 * then do nothing.
+		 */
+		break;
 	}
+
 	return (0);
 }