Easy, ext2online is the answer:
Unless you run into an error like:
ext2online v1.1.19 - 2001/03/18 for EXT2FS 0.5bThat's when you start googling and find that ext2online is not that online and needs the file system to be unmounted you need to run first:
ext2online: group 0, block 2 not reserved
ext2online: unable to resize /dev/mapper/hdapool-homefilesystem
ext2prepare /dev/mapper/hdapool-homefilesystem 23G
Note that the size there is the desired new target for the filesystem.
So you log off, go to a console, umount the file system (unlucky you if is the root file system, in that case you'll need the rescue mode - does D-I have ext2prepare?), run the command.
Now fsck.ext3 the device, mount it and try again ext2online. Now it should work.