file_data: refactor getNextDirty to work recurisvely
file_data: refactor getNextDirty to work recurisvely

Otherwise some traversals of the tree will incorrectly stop at a leaf,
without moving onto the next sibling when looking for dirty nodes.

Issue: KBFS-2091

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