Fix restic backup --one-file-system /
Fix restic backup --one-file-system /

This patch should fix the following panic when trying to backup the
root filesystem with thre --one-file-system flag:

    % restic backup --one-file-system /
    (...)
    panic: item /, device id 2082 not found, allowedDevs: map[/:2082]

kurin/restic,kurin/restic,middelink/restic,restic/restic,middelink/restic,restic/restic