/usr/src/linux-headers-4.2.0-21-generic/fs is in linux-headers-4.2.0-21-generic 4.2.0-21.25~14.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.2.0-21-generic/../linux-headers-4.2.0-21/fs