zfs umount succeeds but prints message "snapshot delimiter '@' is not expected here" to stderr

[youzhongyang]

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	20.04LTS
Kernel Version	5.4.0-26-lowlatency
Architecture	x86_64
OpenZFS Version	2.0.5

Describe the problem you're observing

When trying to unmount a zfs dataset, if a snapshot of the dataset or its descendant dataset is also mounted, the zfs umount command successfully finishes but prints a confusing message like the following to the standard error:

cannot open 'rpool/fs@snap': snapshot delimiter '@' is not expected here

Describe how to reproduce the problem

# zfs create rpool/fs
# zfs create rpool/fs/fs1
# zfs create rpool/fs/fs1/fs2
# zfs snapshot -r rpool/fs@snap
# ls /fs/.zfs/snapshot/snap/ ; zfs umount rpool/fs
fs1
cannot open 'rpool/fs@snap': snapshot delimiter '@' is not expected here
# zfs mount -a
# ls /fs/fs1/.zfs/snapshot/snap/ ; zfs umount rpool/fs
fs2
cannot open 'rpool/fs/fs1@snap': snapshot delimiter '@' is not expected here
# zfs mount -a
# ls /fs/fs1/fs2/.zfs/snapshot/snap/ ; zfs umount rpool/fs
cannot open 'rpool/fs/fs1/fs2@snap': snapshot delimiter '@' is not expected here
# zfs mount -a
# ls /fs/fs1/fs2/.zfs/snapshot/snap/ ; zfs umount rpool/fs ; echo $?
cannot open 'rpool/fs/fs1/fs2@snap': snapshot delimiter '@' is not expected here
0

Include any warning/errors/backtraces from the system logs