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/request.h

@@ -34,6 +34,7 @@ struct search {
 
 	unsigned		insert_data_done:1;
 	unsigned		replace:1;
+	unsigned		insert_collision:1;
 
 	uint16_t		write_prio;