osd: set pg flag INCOMPLETE_CLONES when turning off cache pool

This lets the OSD know that there may be heads without clones even
though cache_mode == NONE.  Fixes #8882.
I think this is correct: If we get a dup, we'll check the first condition and fail if the first message changed the cache_mode from whatever is in committed state;  if there was no change at all, then it's irrelevant.