folder_update_prepper: split out dirData use into own function
folder_update_prepper: split out dirData use into own function

To make cleaning up easier.

Issue: KBFS-3303

keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client