netstack: hardcode hop_limit to 64 in IpScheme for smoltcp 0.13.1

smoltcp 0.13.1 made RawSocket::hop_limit a private field with
no public accessor or setter. The trait methods on IpScheme
cannot usefully read or write the value (anything returned is
just a default, anything written is dropped on the floor).

Return the smoltcp default of 64 from hop_limit() and accept
the write in set_hop_limit() as a no-op. The actual TTL
field in outgoing IPv4 packets is set by smoltcp's default
TTL of 64 when constructing the Repr, so the wire behavior
is unchanged from a fresh smoltcp 0.13.1 stack.

This is the minimal-surface fix that keeps the trait method
implementations compilable on smoltcp 0.13.1 without
introducing a per-socket hop_limit cache. A future
refinement could add a hop_limit field to the SchemeSocket
data type for schemes that need explicit control.

netstack/src/scheme/ip.rs

@@ -53,11 +53,14 @@ impl<'a> SchemeSocket for RawSocket<'a> {
     }
 
     fn hop_limit(&self) -> u8 {
-        self.hop_limit()
+        // smoltcp 0.13.1 made RawSocket::hop_limit a private field
+        // with no public setter; the underlying default is 64 and
+        // outgoing packets use that. Return the default here.
+        64
     }
 
-    fn set_hop_limit(&mut self, hop_limit: u8) {
-        self.set_hop_limit(hop_limit);
+    fn set_hop_limit(&mut self, _hop_limit: u8) {
+        // smoltcp 0.13.1: no public setter; accepted for API compat.
     }
 
     fn new_socket(