core/mount: set Mount.from_proc_self_mountinfo flag before adding default dependencies

core/mount: set Mount.from_proc_self_mountinfo flag before adding default dependencies

When the function mount_setup_new_unit() is called, the corresponding
.mount unit file is not loaded. Hence, Mount.from_fragment is false.
To add default dependencies to e.g. .device units, it is necessary that
the Mount.from_proc_self_mountinfo flag is set. However, the flag was
not set even the unit has information from /proc/self/mountinfo.

Partially fixes #19983.