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 don't think this is right — if through some race a user sends off the same command twice, we don't want one of them to return before the cache mode is actually set. Is there some other way this could happen that we wouldn't want to wait on?