cleanup: fix unused variable + unnecessary mut warnings (7 total)

- router/mod.rs: 4x |mut b| → |b| (&mut [u8] is already mutable)
- scheme/icmp.rs: handle_recvmsg stub params → _file, _how, _flags
- scheme/ip.rs: handle_recvmsg stub params → _file, _how, _flags
- scheme/socket.rs: get_sock_opt default + msg_send ctx → _ prefixed
- scheme/tun.rs: loop name → _name

netstack/src/scheme/socket.rs

@@ -253,9 +253,9 @@ where
 
 fn get_sock_opt(
         &self,
-        file: &SchemeFile<Self>,
-        name: usize,
-        buf: &mut [u8],
+        _file: &SchemeFile<Self>,
+        _name: usize,
+        _buf: &mut [u8],
     ) -> SyscallResult<usize> {
         Err(SyscallError::new(syscall::ENOPROTOOPT))
     }
@@ -448,7 +448,7 @@ where
         fd: usize,
         payload: &mut [u8],
         metadata: &[u64],
-        ctx: &CallerCtx,
+        _ctx: &CallerCtx,
     ) -> SyscallResult<usize> {
         // metadata to Vec<u8>
         let Some(verb) = SocketCall::try_from_raw(metadata[0] as usize) else {