src/msg/async/net_handler.cc: Fix compilation

src/msg/async/net_handler.cc: Fix compilation

On a Cray system I'm working on, it seems that `SO_PRIORITY` is defined,
but `IPTOS_CLASS_CS6` is not. Without this patch, compilation fails at
line 150:

```
r = ::setsockopt(sd, SOL_SOCKET, SO_PRIORITY, &prio, sizeof(prio));
```

because the variable `r` is not defined (originally it is defined inside
the `#ifdef IPTOS_CLASS_CS6` block).

Fixes: https://tracker.ceph.com/issues/42821
Signed-off-by: Carlos Valiente <carlos.valiente@ecmwf.int>