nspawn: inherit the exit status of container

If we get as far as successfully starting the container, nspawn should
inherit the exit status of the child container process as its own.

src/nspawn/nspawn.c

@@ -1696,9 +1696,9 @@ int main(int argc, char *argv[]) {
                 }
 
                 if (status.si_code == CLD_EXITED) {
+                        r = status.si_status;
                         if (status.si_status != 0) {
                                 log_error("Container failed with error code %i.", status.si_status);
-                                r = status.si_status;
                                 break;
                         }