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

Updates #17852

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

util/ringlog/ringlog.go

@@ -4,7 +4,7 @@
 // Package ringlog contains a limited-size concurrency-safe generic ring log.
 package ringlog
 
-import "sync"
+import "tailscale.com/syncs"
 
 // New creates a new [RingLog] containing at most max items.
 func New[T any](max int) *RingLog[T] {
@@ -15,7 +15,7 @@ func New[T any](max int) *RingLog[T] {
 
 // RingLog is a concurrency-safe fixed size log window containing entries of [T].
 type RingLog[T any] struct {
-	mu  sync.Mutex
+	mu  syncs.Mutex
 	pos int
 	buf []T
 	max int