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>
diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 7a1d8dc..441524d 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -2227,6 +2227,9 @@
 		}
 	}
 
+	if (op->insert_collision)
+		return -ESRCH;
+
 	return ret;
 }