oomd: always allow root-owned cgroups to set ManagedOOMPreference
    
    Commit 652a4efb66a ("oomd: loosen the restriction on ManagedOOMPreference")
    made the change to allow ManagedOOMPreference on a cgroup candidate when
    the monitored cgroup and cgroup candidate are owned by the same user.
    
    The commit assumed that this check was sufficient to continue allowing
    ManagedOOMPreference on all cgroups owned by root. However, it caused a
    regression for unprivileged LXD containers where e.g. /sys/fs/cgroup is
    owned by nobody (uid=65534).
    
    Fix this by explicitly allowing the ManagedOOMPreference if uid == 0 in
    oomd_fetch_cgroup_oom_preference().
    
    (cherry picked from commit 89186093485b52ca957d17842fc1f7c87958454a)