tmpfiles: do not mandate `STATX_ATIME` and `STATX_MTIME`

tmpfiles: do not mandate `STATX_ATIME` and `STATX_MTIME`

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.

Correctness analysis
====================

The timestamps produced by the `statx()` call in `opendir_and_stat()`
are only ever used once, in `clean_item_instance()` (lines 3148-3149)
as inputs to `dir_cleanup()`. Convert absent timestamps into
`NSEC_INFINITY` as per the previous commit.

Fixes #41227.