4.4.0-64.85) is not available, but a newer one is (4.4.0-142.168). We redirected you there.
/usr/src/linux-headers-3.13.0-61-lowlatency/net is in linux-headers-3.13.0-61-lowlatency 3.13.0-61.100.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-3.13.0-61-lowlatency/../linux-headers-3.13.0-61/net