Add support for setting SO_SNDBUF for unix domain sockets.

The limits for snd/rcv buffers for unix domain socket is controlled by the
following sysctls on linux

 - net.core.rmem_default
 - net.core.rmem_max
 - net.core.wmem_default
 - net.core.wmem_max

Today in gVisor we do not expose these sysctls but we do support setting the
equivalent in netstack via stack.Options() method. But AF_UNIX sockets in gVisor
can be used without netstack, with hostinet or even without any networking stack
at all. Which means ideally these sysctls need to live as globals in gVisor.

But rather than make this a big change for now we hardcode the limits in the
AF_UNIX implementation itself (which in itself is better than where we were
before) where it SO_SNDBUF was hardcoded to 16KiB. Further we bump the initial
limit to a default value of 208 KiB to match linux from the paltry 16 KiB we use
today.

Updates #5132

PiperOrigin-RevId: 356665498

pkg/sentry/socket/netstack/netstack.go

@@ -855,10 +855,7 @@ func getSockOptSocket(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, fam
 			return nil, syserr.ErrInvalidArgument
 		}
 
-		size, err := ep.SocketOptions().GetSendBufferSize()
-		if err != nil {
-			return nil, syserr.TranslateNetstackError(err)
-		}
+		size := ep.SocketOptions().GetSendBufferSize()
 
 		if size > math.MaxInt32 {
 			size = math.MaxInt32
@@ -1647,13 +1644,6 @@ func setSockOptSocket(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, nam
 			return syserr.ErrInvalidArgument
 		}
 
-		family, _, _ := s.Type()
-		// TODO(gvisor.dev/issue/5132): We currently do not support
-		// setting this option for unix sockets.
-		if family == linux.AF_UNIX {
-			return nil
-		}
-
 		v := usermem.ByteOrder.Uint32(optVal)
 		ep.SocketOptions().SetSendBufferSize(int64(v), true)
 		return nil