samples: bpf: user proper argument index

Use optind as index for argv instead of a hardcoded value.
When the program has options this leads to improper parameter handling.

Fixes: dc378a1ab5 ("samples: bpf: get ifindex from ifname")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Acked-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/xdp1_user.c

@@ -103,7 +103,7 @@ int main(int argc, char **argv)
 		return 1;
 	}
 
-	ifindex = if_nametoindex(argv[1]);
+	ifindex = if_nametoindex(argv[optind]);
 	if (!ifindex) {
 		perror("if_nametoindex");
 		return 1;