exclusive_locks: clear renew-running flag before dropping isLocked (#10413)

On renewal failure the renew goroutine stored isLocked=false before its
deferred renewGoroutineRunning.Store(false) ran. A concurrent RequestLock
interleaving there reacquires the lease (sees isLocked=false), sets
isLocked=true, then its CompareAndSwap on renewGoroutineRunning fails because
the old goroutine's flag is still set — so no replacement renewer starts. The
lock is then held locally with nothing renewing it, and silently expires on
the master after the lease TTL, admitting a second holder.

Clear renewGoroutineRunning before isLocked on the failure path so the
reacquire path always starts a fresh renewer. Builds and vets clean; no
behavior change on the success path.

Claude-Session: https://claude.ai/code/session_01Ks16jnt4S7gdDk8cheQ3xu

weed/wdclient/exclusive_locks/exclusive_locker.go

@@ -93,13 +93,15 @@ func (l *ExclusiveLocker) RequestLock(clientName string) {
 	if l.renewGoroutineRunning.CompareAndSwap(false, true) {
 		// start a goroutine to renew the lease
 		go func() {
-			defer l.renewGoroutineRunning.Store(false)
 			ctx2, cancel2 := context.WithCancel(context.Background())
 			defer cancel2()
 
 			for {
 				if err := l.renewLease(ctx2); err != nil {
 					glog.Warningf("Failed to renew lock %s: %v", l.lockName, err)
+					// clear the running flag before isLocked, so a RequestLock that
+					// reacquires (once isLocked is false) starts a replacement renewer
+					l.renewGoroutineRunning.Store(false)
 					l.isLocked.Store(false)
 					return
 				}