selftests/net: only print passing message in GRO tests when tests pass

gro.c:main no longer erroneously claims a test passes when running as a
sender.

Tested: Ran `gro.sh -t large` to verify the sender no longer prints a
status.

Signed-off-by: Kevin Krakauer <krakauer@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250226192725.621969-3-krakauer@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/gro.c

@@ -1318,11 +1318,13 @@ int main(int argc, char **argv)
 	read_MAC(src_mac, smac);
 	read_MAC(dst_mac, dmac);
 
-	if (tx_socket)
+	if (tx_socket) {
 		gro_sender();
-	else
+	} else {
+		/* Only the receiver exit status determines test success. */
 		gro_receiver();
+		fprintf(stderr, "Gro::%s test passed.\n", testname);
+	}
 
-	fprintf(stderr, "Gro::%s test passed.\n", testname);
 	return 0;
 }