Easier fix might be if you aren't using /usr/src (or /usr/opt) to move the contents of /usr/share/relink into /usr/src (make sure you move it, not copy, so that /usr/share/relink is empty), and then change the /usr/src mount point in /etc/fstab to /usr/share/relink. Reboot the machine and hopefully it works. If you ran out of space during the installation, you may have to repeat sysupgrade to reinstall the 7.8 sets to get all the object files where they belong.