syncs: use TryLock and TryRLock instead of unsafe

The docs say:

Note that while correct uses of TryLock do exist, they are rare,
and use of TryLock is often a sign of a deeper problem in a particular use of mutexes.

Rare code! Or bad code! Who can tell!

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

cmd/tailscale/depaware.txt

@@ -64,7 +64,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
      💣 tailscale.com/net/tshttpproxy                                from tailscale.com/derp/derphttp+
      💣 tailscale.com/paths                                          from tailscale.com/cmd/tailscale/cli+
         tailscale.com/safesocket                                     from tailscale.com/cmd/tailscale/cli+
-     💣 tailscale.com/syncs                                          from tailscale.com/net/interfaces+
+        tailscale.com/syncs                                          from tailscale.com/net/interfaces+
         tailscale.com/tailcfg                                        from tailscale.com/cmd/tailscale/cli+
    W    tailscale.com/tsconst                                        from tailscale.com/net/interfaces
      💣 tailscale.com/tstime/mono                                    from tailscale.com/tstime/rate

cmd/tailscaled/depaware.txt

@@ -228,7 +228,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/safesocket                                     from tailscale.com/client/tailscale+
         tailscale.com/smallzstd                                      from tailscale.com/ipn/ipnserver+
   LD 💣 tailscale.com/ssh/tailssh                                    from tailscale.com/wgengine/netstack
-     💣 tailscale.com/syncs                                          from tailscale.com/control/controlknobs+
+        tailscale.com/syncs                                          from tailscale.com/control/controlknobs+
         tailscale.com/tailcfg                                        from tailscale.com/client/tailscale+
    W    tailscale.com/tsconst                                        from tailscale.com/net/interfaces
         tailscale.com/tstime                                         from tailscale.com/wgengine/magicsock

syncs/locked.go

@@ -2,58 +2,32 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build go1.13 && !go1.19
-// +build go1.13,!go1.19
-
-// This file makes assumptions about the inner workings of sync.Mutex and sync.RWMutex.
-// This includes not just their memory layout but their invariants and functionality.
-// To prevent accidents, it is limited to a known good subset of Go versions.
-
 package syncs
 
 import (
 	"sync"
-	"sync/atomic"
-	"unsafe"
-)
-
-const (
-	mutexLocked = 1
-
-	// sync.Mutex field offsets
-	stateOffset = 0
-
-	// sync.RWMutext field offsets
-	mutexOffset       = 0
-	readerCountOffset = 16
 )
 
-// add returns a pointer with value p + off.
-func add(p unsafe.Pointer, off uintptr) unsafe.Pointer {
-	return unsafe.Pointer(uintptr(p) + off)
-}
-
 // AssertLocked panics if m is not locked.
 func AssertLocked(m *sync.Mutex) {
-	p := add(unsafe.Pointer(m), stateOffset)
-	if atomic.LoadInt32((*int32)(p))&mutexLocked == 0 {
+	if m.TryLock() {
+		m.Unlock()
 		panic("mutex is not locked")
 	}
 }
 
 // AssertRLocked panics if rw is not locked for reading or writing.
 func AssertRLocked(rw *sync.RWMutex) {
-	p := add(unsafe.Pointer(rw), readerCountOffset)
-	if atomic.LoadInt32((*int32)(p)) != 0 {
-		// There are readers present or writers pending, so someone has a read lock.
-		return
+	if rw.TryLock() {
+		rw.Unlock()
+		panic("mutex is not locked")
 	}
-	// No readers.
-	AssertWLocked(rw)
 }
 
 // AssertWLocked panics if rw is not locked for writing.
 func AssertWLocked(rw *sync.RWMutex) {
-	m := (*sync.Mutex)(add(unsafe.Pointer(rw), mutexOffset))
-	AssertLocked(m)
+	if rw.TryRLock() {
+		rw.RUnlock()
+		panic("mutex is not rlocked")
+	}
 }