net/udprelay: use batching.Conn (#16866)

This significantly improves throughput of a peer relay server on Linux.

Server.packetReadLoop no longer passes sockets down the stack. Instead,
packet handling methods return a netip.AddrPort and []byte, which
packetReadLoop gathers together for eventual batched writes on the
appropriate socket(s).

Updates tailscale/corp#31164

Signed-off-by: Jordan Whited <jordan@tailscale.com>

net/batching/conn_linux.go

@@ -384,7 +384,7 @@ func setGSOSizeInControl(control *[]byte, gsoSize uint16) {
 }
 
 // TryUpgradeToConn probes the capabilities of the OS and pconn, and upgrades
-// pconn to a [Conn] if appropriate. A batch size of MinReadBatchMsgsLen() is
+// pconn to a [Conn] if appropriate. A batch size of [IdealBatchSize] is
 // suggested for the best performance.
 func TryUpgradeToConn(pconn nettype.PacketConn, network string, batchSize int) nettype.PacketConn {
 	if runtime.GOOS != "linux" {
@@ -457,6 +457,4 @@ func MinControlMessageSize() int {
 	return controlMessageSize
 }
 
-func MinReadBatchMsgsLen() int {
-	return 128
-}
+const IdealBatchSize = 128