sys-apps/systemd: add upstream patch

[tormath1]

This patch is a bit adapted from the upstream to remove kernel 6.15 limitation (open_tree_attr)

Testing done

Locally built:

$ sudo emerge -v systemd
 * Applying 41875.patch ...                                                                                                                                                           [ ok ]
...
>>> Completed (1 of 1) sys-apps/systemd-257.9::portage-stable

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• CI: https://jenkins.flatcar.org/job/container/job/packages_all_arches/139/cldsv/

EDIT: For now it's failing:

May 05 12:56:08 localhost systemd[1]: Starting systemd-sysext.service - Merge System Extension Images into /usr/ and /opt/...
May 05 12:56:08 localhost (sd-merge)[1234]: Using extensions 'containerd-flatcar.raw', 'docker-flatcar.raw', 'oem-qemu.raw'.
May 05 12:56:08 localhost (sd-merge)[1234]: Failed to move mount /opt/foo to /run/systemd/sysext/submounts/opt/foo: Invalid argument
May 05 12:56:08 localhost systemd-sysext[1214]: Failed to merge hierarchies
May 05 12:56:08 localhost systemd[1]: systemd-sysext.service: Main process exited, code=exited, status=1/FAILURE
May 05 12:56:08 localhost systemd[1]: systemd-sysext.service: Failed with result 'exit-code'.
May 05 12:56:08 localhost systemd[1]: Failed to start systemd-sysext.service - Merge System Extension Images into /usr/ and /opt/.

Related to: flatcar/Flatcar#2111