fix: apply suggestion from review comment

#1842 (comment)

Author: jmonticelli

io/zenoh-links/zenoh-link-udp/src/multicast.rs

@@ -323,29 +323,23 @@ impl LinkManagerMulticastUdp {
 
         // If TTL is specified, add set the socket's TTL
         if let Some(ttl_str) = config.get(UDP_MULTICAST_TTL) {
-            let ttl = match ttl_str.parse::<u32>() {
-                Ok(ttl) => ttl,
-                Err(e) => bail!("Can not parse TTL '{}' to a u32: {}", ttl_str, e),
-            };
-
             match &local_addr {
                 IpAddr::V4(_) => {
-                    ucast_sock
-                        .set_multicast_ttl_v4(ttl)
-                        .map_err(|e| zerror!("{}: {}", mcast_addr, e))?;
+                    let ttl = match ttl_str.parse::<u32>() {
+                        Ok(ttl)  => ttl,
+                        Err(e) => bail!("Can not parse TTL '{}' to a u32: {}", ttl_str, e),
+                    };
+
+                    ucast_sock.set_multicast_ttl_v4(ttl).map_err(|e| {
+                        zerror!("Can not set multicast TTL {} on {}: {}", ttl, mcast_addr, e)
+                    })?;
                 }
-                IpAddr::V6(_) => match zenoh_util::net::get_index_of_interface(local_addr) {
-                    Ok(_) => {
-                        tracing::warn!(
-                            "set_multicast_ttl_v4 on v6 socket (may have no effect): {}",
+                IpAddr::V6(_) => {
+                    tracing::warn!(
+                            "UDP multicast hop limit not supported for v6 socket: {}. See https://github.com/rust-lang/rust/pull/138744.",
                             mcast_addr
                         );
-                        ucast_sock
-                            .set_multicast_ttl_v4(ttl)
-                            .map_err(|e| zerror!("{}: {}", mcast_addr, e))?
-                    }
-                    Err(e) => bail!("{}: {}", mcast_addr, e),
-                },
+                }
             }
         }