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

Updates #17852

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

util/expvarx/expvarx.go

@@ -7,9 +7,9 @@ package expvarx
 import (
 	"encoding/json"
 	"expvar"
-	"sync"
 	"time"
 
+	"tailscale.com/syncs"
 	"tailscale.com/types/lazy"
 )
 
@@ -20,7 +20,7 @@ type SafeFunc struct {
 	limit  time.Duration
 	onSlow func(time.Duration, any)
 
-	mu       sync.Mutex
+	mu       syncs.Mutex
 	inflight *lazy.SyncValue[any]
 }