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

Updates #17852

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

net/dns/resolver/debug.go

@@ -8,12 +8,12 @@ import (
 	"html"
 	"net/http"
 	"strconv"
-	"sync"
 	"sync/atomic"
 	"time"
 
 	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health"
+	"tailscale.com/syncs"
 )
 
 func init() {
@@ -39,7 +39,7 @@ func init() {
 var fwdLogAtomic atomic.Pointer[fwdLog]
 
 type fwdLog struct {
-	mu  sync.Mutex
+	mu  syncs.Mutex
 	pos int // ent[pos] is next entry
 	ent []fwdLogEntry
 }

net/dns/resolver/forwarder.go

@@ -37,6 +37,7 @@ import (
 	"tailscale.com/net/netx"
 	"tailscale.com/net/sockstats"
 	"tailscale.com/net/tsdial"
+	"tailscale.com/syncs"
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/nettype"
@@ -231,7 +232,7 @@ type forwarder struct {
 	ctx       context.Context    // good until Close
 	ctxCancel context.CancelFunc // closes ctx
 
-	mu sync.Mutex // guards following
+	mu syncs.Mutex // guards following
 
 	dohClient map[string]*http.Client // urlBase -> client
 

net/dns/resolver/tsdns.go

@@ -214,7 +214,7 @@ type Resolver struct {
 	closed chan struct{}
 
 	// mu guards the following fields from being updated while used.
-	mu           sync.Mutex
+	mu           syncs.Mutex
 	localDomains []dnsname.FQDN
 	hostToIP     map[dnsname.FQDN][]netip.Addr
 	ipToHost     map[netip.Addr]dnsname.FQDN