selftests/bpf: Omit nodad flag when adding addresses to loopback

Setting IFA_F_NODAD flag for IPv6 addresses to add to loopback is
unnecessary. Duplicate Address Detection does not happen on loopback
device.

Also, passing 'nodad' flag to 'ip address' breaks libbpf CI, which runs in
an environment with BusyBox implementation of 'ip' command, that doesn't
understand this flag.

Fixes: 0ab5539f ("selftests/bpf: Tests for BPF_SK_LOOKUP attach point")
Reported-by: NAndrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: NJakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Tested-by: NAndrii Nakryiko <andrii@fb.com>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200730125325.1869363-1-jakub@cloudflare.com

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

@@ -1290,8 +1290,8 @@ static void run_tests(struct test_sk_lookup *skel)
 static int switch_netns(void)
 {
 	static const char * const setup_script[] = {
-		"ip -6 addr add dev lo " EXT_IP6 "/128 nodad",
-		"ip -6 addr add dev lo " INT_IP6 "/128 nodad",
+		"ip -6 addr add dev lo " EXT_IP6 "/128",
+		"ip -6 addr add dev lo " INT_IP6 "/128",
 		"ip link set dev lo up",
 		NULL,
 	};