wgengine/magicsock: set UDP socket buffer sizes to 7MB

- At high data rates more buffer space is required in order to avoid
  packet loss during any cause of delay.
- On slower machines more buffer space is required in order to avoid
  packet loss while decryption & tun writing is underway.
- On higher latency network paths more buffer space is required in order
  to overcome BDP.
- On Linux set with SO_*BUFFORCE to bypass net.core.{r,w}mem_max.
- 7MB is the current default maximum on macOS 12.6
- Windows test is omitted, as Windows does not support getsockopt for
  these options.

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/magicsock_default.go

@@ -10,8 +10,15 @@ package magicsock
 import (
 	"errors"
 	"io"
+
+	"tailscale.com/types/logger"
+	"tailscale.com/types/nettype"
 )
 
 func (c *Conn) listenRawDisco(family string) (io.Closer, error) {
 	return nil, errors.New("raw disco listening not supported on this OS")
 }
+
+func trySetSocketBuffer(pconn nettype.PacketConn, logf logger.Logf) {
+	portableTrySetSocketBuffer(pconn, logf)
+}