I tried to mount a formerly readonly mounted filesystem read-writeable:

mount -o remount,rw /mountpoint

Unfortunately it did not work:

mount: /mountpoint not mounted already, or bad option

dmesg reports:

[2570543.520449] EXT4-fs (dm-0): Couldn't remount RDWR because of unprocessed orphan inode list.  Please umount/remount instead

A umount does not work, too:

umount /mountpoint
umount: /mountpoint: device is busy.
    (In some cases useful info about processes that use
     the device is found by lsof(8) or fuser(1))

Unfortunately neither lsof of fuser don't show any process accessing something located under the mount point.

So - how can I clean up this unprocessed orphan list to be able to mount the filesystem again without rebooting the computer?