systemd-tmpfiles: exclude "lost+found" from "D" lines

systemd version the issue has been seen with
systemd v241-1 (from Debian buster repositories)
Same issue with current git checkout (commit 1eacc47)

Used distribution
Debian 10 x64

Expected behaviour you didn't see
lost+found not getting touched by systemd-tmpfiles

Unexpected behaviour you saw
lost+found gets cleaned up

Steps to reproduce the problem
Debian systems truncate their /tmp at boot via systemd-tmpfiles-setup.service, configured as

D /tmp 1777 root root -

On our machines /tmp is a separate partition (on LVM, but that should not matter) with ext4 so it is its own mountpoint and contains a lost+found directory.
Since the configurations states TRUNCATE_DIRECTORY we run into this FIXME: https://github.com/systemd/systemd/blob/master/src/tmpfiles/tmpfiles.c#L2143

I replaced the FIXME and a similar one at https://github.com/systemd/systemd/blob/master/src/tmpfiles/tmpfiles.c#L2118 with the call to dir_cleanup() and it seems to fix our problem. But I do not know if that breaks anything else or how to properly handle the cutoff parameter in this case.

Do you have any pointers how to tackle this? I would be happy to submit a pull request.

Start in src/tmpfiles/tmpfiles.c at the FIXME locations around lines 2118 and 2143, and read how D lines invoke directory cleanup, including the cutoff parameter. Use the reported ext4 /tmp reproduction to verify the behavior. Done means systemd-tmpfiles leaves lost+found untouched while still applying the requested cleanup elsewhere.