core/device: ignore ID_PROCESSING udev property on enumerate

core/device: ignore ID_PROCESSING udev property on enumerate

This partially reverts the commit 405be62f05d76f1845f347737b5972158c79dd3e
"tree-wide: refuse enumerated device with ID_PROCESSING=1".

Otherwise, when systemd-udev-trigger.service is (re)started just before
daemon-reexec, which can be easily happen on systemd package update, then
udev database files for many devices may have ID_PROCESSING=1 property,
thus devices may not be enumerated on daemon-reexec. That causes many
units especially mount units being deactivated after daemon-reexec.

Fixes #35329.