try using the supersu app to "make a system app" or "install into system" because this build of android uses the /system method and it would be less difficult than trying to "unroot" via the supersu app and then trying to debug a systemless install. after supersu finishes "change to system app" then use supersu to update the binary. that has so far worked for me in the 7.1 build. issue i was having with mounting /system r/w was such that i needed to drop it for awhile.
edit: additionally install busybox i use stephen erikson's installer for busybox and also i use joey krim's root checker to verify certain aspects of a root install, also gives hints on what is wrong.
edit: additionally install busybox i use stephen erikson's installer for busybox and also i use joey krim's root checker to verify certain aspects of a root install, also gives hints on what is wrong.