bcachefs: Fix promotes

The recent work to fix data moves w.r.t. durability broke promotes,
because the caused us to bail out when the extent minus pointers being
dropped still has enough pointers to satisfy the current number of
replicas.

Disable this check when we're adding cached replicas.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/data_update.c

@@ -587,7 +587,8 @@ int bch2_data_update_init(struct btree_trans *trans,
 	 * Increasing replication is an explicit operation triggered by
 	 * rereplicate, currently, so that users don't get an unexpected -ENOSPC
 	 */
-	if (durability_have >= io_opts.data_replicas) {
+	if (!(m->data_opts.write_flags & BCH_WRITE_CACHED) &&
+	    durability_have >= io_opts.data_replicas) {
 		m->data_opts.kill_ptrs |= m->data_opts.rewrite_ptrs;
 		m->data_opts.rewrite_ptrs = 0;
 		/* if iter == NULL, it's just a promote */