Unable to unmount idle filesystem on 6.2

[Darren Pilgrim]

I'm unable to unmount an idle filesystem (or even drop it to
read-only):

# mount
/dev/da0s1a on / (ufs, local, noatime)
devfs on /dev (devfs, local)
/dev/da0s1d on /var (ufs, local, noatime, soft-updates)
/dev/da0s1e on /usr (ufs, local, noatime, soft-updates)
/dev/da0s1fp1 on /usr/obj (ufs, asynchronous, local, noatime)
/dev/da0s1fp2 on /usr/ports (ufs, local, soft-updates)
/dev/da0s1fp3 on /usr/src (ufs, local, soft-updates)
/dev/da0s2d on /data (ufs, local, noatime, soft-updates)

# fstat -f /usr/ports
USER     CMD          PID   FD MOUNT      INUM MODE         SZ|DV R/W

# umount /usr/ports
umount: unmount of /usr/ports failed: Device busy

# umount -f /usr/ports
umount: unmount of /usr/ports failed: Device busy

# mount -o ro /usr/ports
mount: /dev/da0s1fp2: Operation not permitted

# uname -r
6.2-RELEASE-p8