Fix sharing of ext storage mount points

When sharing an ext storage mount point, it will now use the name of the
mount point instead of an empty string for the target path.

Fixes #5347 
This fix is compatible with stable6 as long as #8431 gets fixed there.

There is an additional issue: when navigating into the folder the user doesn't have rename or delete permissions. Needs further research.

@schiesbn @icewind1991