bcache: Don't use op->insert_collision

When we convert bch_btree_insert() to bch_btree_map_leaf_nodes(), we
won't be passing struct btree_op to bch_btree_insert() anymore - so we
need a different way of returning whether there was a collision (really,
a replace collision).

Signed-off-by: Kent Overstreet <kmo@daterainc.com>

drivers/md/bcache/btree.c

@@ -2227,6 +2227,9 @@ int bch_btree_insert(struct btree_op *op, struct cache_set *c,
 		}
 	}
 
+	if (op->insert_collision)
+		return -ESRCH;
+
 	return ret;
 }