Link2sd, partition unmounted itself, can't mount back
I dont use Sony Xperia M2 but I had similar problem with Link2SD after upgrade to Android version 6.0.1 on my Samsung NOTE4 N910C.
The reason is - from my point of view - fact, that Link2sd is not longer supported especially for new versions of Android.
I used Apps2SD: All in One Tool[ROOT] (cost aprox 3,5 USD in my Country). Link
-> Look here

Sorry it is in Polish language but of course You can switch to Your language.
This app is still under develop and has more features than Link2SD like working properly recreation of mounting script