leaked mountpoints after umount

Sometimes after umounting a filesystem (maybe resulting in an error
condition) it will remain listed in the mount table:

dalki# mount -v
/dev/da0s1a on / (ufs, local, fsid ba3af642adf91c2a)
devfs on /dev (devfs, local, fsid 00ff000505000000)
/dev/da0s1e on /d (ufs, local, soft-updates, fsid bb3af6420012cc6a)
/dev/da0s1d on /usr (ufs, local, soft-updates, fsid bb3af64216b6e4d0)
pfi.true.lv:/sdc1/dosirak on /data (nfs, fsid 01ff000202000000)
haessal:/c/test on /haessal (nfs, read-only, fsid c7ff560202000000)

dalki# umount -f c7ff560202000000
umount: unmount of /haessal failed: No such file or directory
umount: retrying using path instead of file system ID
umount: unmount of /haessal failed: No such file or directory

dalki# mount -v
/dev/da0s1a on / (ufs, local, fsid ba3af642adf91c2a)
devfs on /dev (devfs, local, fsid 00ff000505000000)
/dev/da0s1e on /d (ufs, local, soft-updates, fsid bb3af6420012cc6a)
/dev/da0s1d on /usr (ufs, local, soft-updates, fsid bb3af64216b6e4d0)
pfi.true.lv:/sdc1/dosirak on /data (nfs, fsid 01ff000202000000)
haessal:/c/test on /haessal (nfs, read-only, fsid c7ff560202000000)

It is not actually mounted (/haessal is the underlying UFS directory),
but it cannot be purged from the mount table.

Kris