lib9p: fix Tattach ref count bug

src/lib9p/srv.c

diff --git a/src/lib9p/srv.c b/src/lib9p/srv.c
index 52d4bcc..7393b3c 100644
--- a/src/lib9p/srv.c
+++ b/src/lib9p/srv.c
@@ -227,7 +227,7 @@
 	r->fid->uid = estrdup9p(r->ifcall.uname);
 	if(srv->tree){
 		r->fid->file = srv->tree->root;
-		/* BUG? incref(r->fid->file) ??? */
+		incref(&r->fid->file->ref);
 		r->ofcall.qid = r->fid->file->dir.qid;
 		r->fid->qid = r->ofcall.qid;
 	}