Setup shared mounts for the correct user when setting up the filesystem

Currently the mountpoint setup logic for files_shared will always setup the shared mountpoints for the logged in user, no matter what user the filesystem is being setup for.

Besides resulting in the filesystem not being in the state we expect it to this causes the shared mounts for the logged in user to be setup multiple times leading to the `(2) (2) (2)` explosion.

cc @DeepDiver1975 @PVince81 @schiesbn 
https://github.com/owncloud/core/pull/11945/files#diff-d802c5d8dc58cc57f6375a3a4f32b2bfR414