vfs.usermount allows users to do arbitrary unmounts

[Yuri Pankov]

Hi,

Going through https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=232021, 
I noticed that unmount(2) will not fail with EPERM as expected on mounts 
that don't belong to the user, and which were not mounted by that user, 
e.g.:

# zfs create data/test
# ls -ld /data/test
drwxr-xr-x  2 root  wheel  2 Oct  7 21:55 /data/test
$ umount /data/test; echo $?
0

Even if vfs.usermount isn't great from security perspective, is this 
expected?