Setup file locking storage wrapped in setup hook

This fixes an issue when using file locking + encryption and try to list
a shared mount in the web UI.

Using the setup hook prevents having setupFS() be called twice in a row
and mess up with the mount points.

Please review @schiesbn @icewind1991 @DeepDiver1975 

Fixes https://github.com/owncloud/core/issues/9222
See above ticket for my analysis of the issue.