kern/57467: zfs cannot umount snapshots once access it via /usr/.zfs/shapshots/ ....

[nervoso%k1.com.br@localhost]

>Number:         57467
>Category:       kern
>Synopsis:       zfs cannot umount snapshots once access it via /usr/.zfs/shapshots/ ....
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 12 23:15:00 +0000 2023
>Originator:     sergio lenzi
>Release:        10.99.4
>Organization:
k1 sistemas
>Environment:
NetBSD lenzi.lenzicasa 10.99.4 NetBSD 10.99.4 (LZT) #14: Sun Jun 11 12:23:15 -03 2023  NetBSD@devel.lenzicasa:/home/NetBSD/BUILD/10/amd64/OBJ/sys/arch/amd64/compile/GENERIC amd64
>Description:
when access a snapsht for example:
cd /usr/.zfs/snapshots/2023-06-02_23.59.00--7d.
the system accepts by mounting a zfs node into that position...
I can access all the files ... do rsync ...  etc.. 
but once I leave the directory, the mountpoint remains, and
there is NO WAY to umount it... umount /usr/.zfs/.... => operation not permitted even on root
>How-To-Repeat:
on a NetBSD 10.99.4 system running zfs...
zfs list -t snap
choose snapshot to mount
cd /path/.zfs/snapshot/......  
cp some files to /tmp or whatever
exit directory...


the snapshot remains mounted and there is no way to umount it unless reboot
>Fix: