net/sockopts,wgengine/magicsock: export socket buffer sizing logic (#16909)

For eventual use by net/udprelay.Server

Updates tailscale/corp#31164

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

net/sockopts/sockopts_default.go

@@ -0,0 +1,21 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux
+
+package sockopts
+
+import (
+	"tailscale.com/types/nettype"
+)
+
+// SetBufferSize sets pconn's buffer to size for direction. size may be silently
+// capped depending on platform.
+//
+// errForce is only relevant for Linux, and will always be nil otherwise,
+// but we maintain a consistent cross-platform API.
+//
+// If pconn is not a [*net.UDPConn], then SetBufferSize is no-op.
+func SetBufferSize(pconn nettype.PacketConn, direction BufferDirection, size int) (errForce error, errPortable error) {
+	return nil, portableSetBufferSize(pconn, direction, size)
+}