samples/bpf: Fix Segmentation fault for xdp_redirect command

A Segmentation fault error is caused when the following command
is executed.

$ sudo ./samples/bpf/xdp_redirect lo
Segmentation fault

This command is missing a device <IFNAME|IFINDEX> as an argument, resulting
in out-of-bounds access from argv.

If the number of devices for the xdp_redirect parameter is not 2,
we should report an error and exit.

Fixes: 24251c26 ("samples/bpf: add option for native and skb mode for redirect apps")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210616042324.314832-1-wanghai38@huawei.com

samples/bpf/xdp_redirect_user.c

@@ -130,7 +130,7 @@ int main(int argc, char **argv)
 	if (!(xdp_flags & XDP_FLAGS_SKB_MODE))
 		xdp_flags |= XDP_FLAGS_DRV_MODE;
 
-	if (optind == argc) {
+	if (optind + 2 != argc) {
 		printf("usage: %s <IFNAME|IFINDEX>_IN <IFNAME|IFINDEX>_OUT\n", argv[0]);
 		return 1;
 	}