syncs: add Mutex/RWMutex alias/wrappers for future mutex debugging

Updates #17852

Change-Id: I477340fb8e40686870e981ade11cd61597c34a20
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/netcheck/netcheck.go

@@ -235,7 +235,7 @@ type Client struct {
 	testEnoughRegions      int
 	testCaptivePortalDelay time.Duration
 
-	mu       sync.Mutex            // guards following
+	mu       syncs.Mutex           // guards following
 	nextFull bool                  // do a full region scan, even if last != nil
 	prev     map[time.Time]*Report // some previous reports
 	last     *Report               // most recent report
@@ -597,7 +597,7 @@ type reportState struct {
 	stopProbeCh chan struct{}
 	waitPortMap sync.WaitGroup
 
-	mu       sync.Mutex
+	mu       syncs.Mutex
 	report   *Report                            // to be returned by GetReport
 	inFlight map[stun.TxID]func(netip.AddrPort) // called without c.mu held
 	gotEP4   netip.AddrPort