how can i mount system at 5.1 (UL,Root,Stock-Rom)?

Hey there browsing with all those ads is a real pain, therefore i wanted to edit the hosts-file since adaway didn't worked.

Therefore i wanted to use the content of this page and add it to my hosts-file:
http://someonewhocares.org/hosts/

problem is, i can't mount /system with total commander although root is working (flashtool installed busybox fine as the root popup comes up and asking for root).

the bootloader is unlocked and via adb shell in root mount i also get:

Code:

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root@C6903:/ # mount -o remount,rw /system
mount -o remount,rw /system
mount: Operation not permitted

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is there another way? can i somehow flash the hosts-file trough recovery?