folder_block_ops: remove the last direct dblock assumptions
folder_block_ops: remove the last direct dblock assumptions

From `searchForNodesInDirLocked` and
`fastForwardDirAndChildrenLocked`.

Issue: KBFS-3302

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