change root location to /system/xbin android
I am also facing the same problem.

I got a workaround that changes the installation scripts not to save the su image to / data, but to / system, and by modifying the su startup script to mount su into / system as well. However this method only works by installing su via recovery (using TWRP_3.0.2).

I am still looking for an alternative to do within the system

I changed the scripts from SuperSU-v2.82 tool 

https://forum.xda-developers.com/apps/supersu