In natm_usr_abort(), replace existing call to natm_usr_shutdown() with

natm_usr_detach(), which actually does the right thing.  This code has
never worked properly, but also was never invoked since we only abort
connections associated with listen sockets, and netnam doesn't support
listen sockets.

MFC after:	3 months

sys/netnatm/natm.c

@@ -335,7 +335,7 @@ natm_usr_control(struct socket *so, u_long cmd, caddr_t arg,
 static void
 natm_usr_abort(struct socket *so)
 {
-    natm_usr_shutdown(so);
+    natm_usr_detach(so);
 }
 
 static int