mirage-unix: increase TCP buffer size

On `AMD Ryzen 9 7950X` using Fedora39 and OCaml 4.14.1 testing with
iperf on localhost, with 4K buffer:
```
[  1] 0.00-10.00 sec  31.1 GBytes  26.7 Gbits/sec
```

With 64KiB buffer:
```
[  1] 0.00-10.00 sec  49.9 GBytes  42.9 Gbits/sec
```

The max size for various TSO/GSO/GRO packet offloads in Linux is 64KiB,
so going beyond that is unlikely to be useful [1].

[1]: although there are plans for larger than 64KiB offloads https://netdevconf.info/0x15/slides/35/BIG%20TCP.pdf

Signed-off-by: Edwin Török <[email protected]>

src/stack-unix/tcp_socket.ml

@@ -19,7 +19,7 @@ let disconnect _ =
   return_unit
 
 let read fd =
-  let buflen = 4096 in
+  let buflen = 65536 in
   let buf = Cstruct.create buflen in
   Lwt.catch (fun () ->
       Lwt_cstruct.read fd buf