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 in the case of a dup the first p->cache_mode will fail (p is the committed pg_pool_t).  The condition is supposed to be "if the new value matches the committed value AND there is no change to that field pending" (either no pool update at all pending, or there is a pool update but this field is not changed as part of it)