folder_branch_ops: don't fast forward while dirty
folder_branch_ops: don't fast forward while dirty

Fast-forward while dirty can cause the device to unlink unflushed
nodes that have been made while disconnected, but it won't actually
delete the dir entries from the dirty blocks.  So when the flush
finally happens, the journal will reference non-existent blocks, and
all hell will break loose during CR.

Issue: keybase/client#9566

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