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

Updates #17852

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

net/wsconn/wsconn.go

@@ -12,11 +12,11 @@ import (
 	"math"
 	"net"
 	"os"
-	"sync"
 	"sync/atomic"
 	"time"
 
 	"github.com/coder/websocket"
+	"tailscale.com/syncs"
 )
 
 // NetConn converts a *websocket.Conn into a net.Conn.
@@ -102,7 +102,7 @@ type netConn struct {
 	reading           atomic.Bool
 	afterReadDeadline atomic.Bool
 
-	readMu sync.Mutex
+	readMu syncs.Mutex
 	// eofed is true if the reader should return io.EOF from the Read call.
 	//
 	// +checklocks:readMu