fd-util: skip to check mount ID if kernel is too old and /proc is not mounted

fd-util: skip to check mount ID if kernel is too old and /proc is not mounted

Now, dir_fd_is_root() is heavily used in chaseat(), which is used at
various places. If the kernel is too old and /proc is not mounted, then
there is no way to get the mount ID of a directory. In that case, let's
silently skip the mount ID check.

Fixes https://github.com/systemd/systemd/pull/27299#issuecomment-1511403680.