samples/bpf: Fix the error return code of xdp_redirect's main()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

If bpf_map_update_elem() failed, main() should return a negative error.

Fixes: 832622e6bd18 ("xdp: sample program for new bpf_redirect helper")
Signed-off-by: Wang Hai <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/xdp_redirect_user.c

···
 	poll_stats(2, ifindex_out);
 
 out:
-	return 0;
+	return ret;
 }