selftests/bpf: Fix flaky fib_lookup test

There is a report that fib_lookup test is flaky when running in parallel.
A symptom of slowness or delay. An example:

Testing IPv6 stale neigh
set_lookup_params:PASS:inet_pton(IPV6_IFACE_ADDR) 0 nsec
test_fib_lookup:PASS:bpf_prog_test_run_opts 0 nsec
test_fib_lookup:FAIL:fib_lookup_ret unexpected fib_lookup_ret: actual 0 != expected 7
test_fib_lookup:FAIL:dmac not match unexpected dmac not match: actual 1 != expected 0
dmac expected 11:11:11:11:11:11 actual 00:00:00:00:00:00

[ Note that the "fib_lookup_ret unexpected fib_lookup_ret actual 0 ..."
  is reversed in terms of expected and actual value. Fixing in this
  patch also. ]

One possibility is the testing stale neigh entry was marked dead by the
gc (in neigh_periodic_work). The default gc_stale_time sysctl is 60s.
This patch increases it to 15 mins.

It also:

- fixes the reversed arg (actual vs expected) in one of the
  ASSERT_EQ test
- removes the nodad command arg when adding v4 neigh entry which
  currently has a warning.

Fixes: 168de02335 ("selftests/bpf: Add bpf_fib_lookup test")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230309060244.3242491-1-martin.lau@linux.dev

tools/testing/selftests/bpf/prog_tests/fib_lookup.c

@@ -54,11 +54,19 @@ static int setup_netns(void)
 	SYS(fail, "ip link add veth1 type veth peer name veth2");
 	SYS(fail, "ip link set dev veth1 up");
 
+	err = write_sysctl("/proc/sys/net/ipv4/neigh/veth1/gc_stale_time", "900");
+	if (!ASSERT_OK(err, "write_sysctl(net.ipv4.neigh.veth1.gc_stale_time)"))
+		goto fail;
+
+	err = write_sysctl("/proc/sys/net/ipv6/neigh/veth1/gc_stale_time", "900");
+	if (!ASSERT_OK(err, "write_sysctl(net.ipv6.neigh.veth1.gc_stale_time)"))
+		goto fail;
+
 	SYS(fail, "ip addr add %s/64 dev veth1 nodad", IPV6_IFACE_ADDR);
 	SYS(fail, "ip neigh add %s dev veth1 nud failed", IPV6_NUD_FAILED_ADDR);
 	SYS(fail, "ip neigh add %s dev veth1 lladdr %s nud stale", IPV6_NUD_STALE_ADDR, DMAC);
 
-	SYS(fail, "ip addr add %s/24 dev veth1 nodad", IPV4_IFACE_ADDR);
+	SYS(fail, "ip addr add %s/24 dev veth1", IPV4_IFACE_ADDR);
 	SYS(fail, "ip neigh add %s dev veth1 nud failed", IPV4_NUD_FAILED_ADDR);
 	SYS(fail, "ip neigh add %s dev veth1 lladdr %s nud stale", IPV4_NUD_STALE_ADDR, DMAC);
 
@@ -158,7 +166,7 @@ void test_fib_lookup(void)
 		if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
 			continue;
 
-		ASSERT_EQ(tests[i].expected_ret, skel->bss->fib_lookup_ret,
+		ASSERT_EQ(skel->bss->fib_lookup_ret, tests[i].expected_ret,
 			  "fib_lookup_ret");
 
 		ret = memcmp(tests[i].dmac, fib_params->dmac, sizeof(tests[i].dmac));