net/selftests: Add loopback link local route for self-connect

self-connect-ipv6 got slightly flaky on netdev:
> # timeout set to 120
> # selftests: net/tcp_ao: self-connect_ipv6
> # 1..5
> # # 708[lib/setup.c:250] rand seed 1742872572
> # TAP version 13
> # # 708[lib/proc.c:213]    Snmp6            Ip6OutNoRoutes: 0 => 1
> # not ok 1 # error 708[self-connect.c:70] failed to connect()
> # ok 2 No unexpected trace events during the test run
> # # Planned tests != run tests (5 != 2)
> # # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:1
> ok 1 selftests: net/tcp_ao: self-connect_ipv6

I can not reproduce it on my machines, but judging by "Ip6OutNoRoutes"
there is no route to the local_addr (::1).

Looking at the kernel code, I see that kernel does add link-local
address automatically in init_loopback(), but that is called from
ipv6 notifier block. So, in turn the userspace that brought up
the loopback interface may see rtnetlink ACK earlier than
addrconf_notify() does it's job (at least, on a slow VM such as netdev).
Probably, for ipv4 it's the same, judging by inetdev_event().

The fix is quite simple: set the link-local route straight after
bringing the loopback interface. That will make it synchronous.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20250402-tcp-ao-selfconnect-flake-v1-1-8388d629ef3d@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/tcp_ao/self-connect.c

@@ -16,6 +16,9 @@ static void __setup_lo_intf(const char *lo_intf,
 
 	if (link_set_up(lo_intf))
 		test_error("Failed to bring %s up", lo_intf);
+
+	if (ip_route_add(lo_intf, TEST_FAMILY, local_addr, local_addr))
+		test_error("Failed to add a local route %s", lo_intf);
 }
 
 static void setup_lo_intf(const char *lo_intf)