net/ktimeout: add a package to set TCP user timeout

Setting a user timeout will be a more practical tuning knob for a number
of endpoints, this provides a way to set it.

Updates tailscale/corp#17587

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

net/ktimeout/ktimeout_linux.go

@@ -0,0 +1,15 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package ktimeout
+
+import (
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+// SetUserTimeout sets the TCP_USER_TIMEOUT option on the given file descriptor.
+func SetUserTimeout(fd uintptr, timeout time.Duration) error {
+	return unix.SetsockoptInt(int(fd), unix.SOL_TCP, unix.TCP_USER_TIMEOUT, int(timeout/time.Millisecond))
+}