tmpfiles: do not require `STATX_ATIME` (#41232)

Timestamps are not guaranteed to be set by `statx()`, and their presence
should not be asserted as a proxy to judge the kernel version. In
particular, `STATX_ATIME` is omitted from the return when querying a
file on a `noatime` superblock, causing spurious errors from tmpfiles:

```console
# SYSTEMD_LOG_LEVEL=debug systemd-tmpfiles --clean
<...>
Running clean action for entry X /var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-*/tmp
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-prometheus-smartctl-exporter.service-GKguQK/tmp) failed: Protocol driver not attached
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-systemd-logind.service-k8j52T/tmp) failed: Protocol driver not attached
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-irqbalance.service-7RJkev/tmp) failed: Protocol driver not attached
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-chronyd.service-8hkO5G/tmp) failed: Protocol driver not attached
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-dbus-broker.service-6P6LVl/tmp) failed: Protocol driver not attached
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-nginx.service-B5HX8B/tmp) failed: Protocol driver not attached
Running clean action for entry x /var/tmp/systemd-private-94cc8a77688e497f96d5b9019e66ed6f-*
Running clean action for entry q /var/tmp
statx() does not support 'STATX_ATIME' mask (running on an old kernel?)
statx(/var/tmp) failed: Protocol driver not attached
<...>
```

Additionally, refactor `dir_cleanup()` slightly for self-consistency to
make
it evident that the `NSEC_INFINITY` transformation is correct.

Fixes #41227.

diff --cc src/tmpfiles/tmpfiles.c
Simple merge