Using the same synchornized to guard _cancelled status, which need recursive lock

DO NOT USE SD_LOCK (os_unfair_lock), which not support recursive

SDWebImage/Core/SDWebImageManager.h

@@ -29,6 +29,9 @@ typedef void(^SDInternalCompletionBlock)(UIImage * _Nullable image, NSData * _Nu
  */
 - (void)cancel;
 
+/// Whether the operation has been cancelled.
+@property (nonatomic, assign, readonly, getter=isCancelled) BOOL cancelled;
+
 /**
  The cache operation from the image cache query
  */

SDWebImage/Core/SDWebImageManager.m

@@ -803,12 +803,20 @@ static id<SDImageLoader> _defaultImageLoader;
 
 @implementation SDWebImageCombinedOperation
 
-- (void)cancel {
+- (BOOL)isCancelled {
+    // Need recursive lock (user's cancel block may check isCancelled), do not use SD_LOCK
     @synchronized (self) {
-        if (self.isCancelled) {
+        return _cancelled;
+    }
+}
+
+- (void)cancel {
+    // Need recursive lock (user's cancel block may check isCancelled), do not use SD_LOCK
+    @synchronized(self) {
+        if (_cancelled) {
             return;
         }
-        self.cancelled = YES;
+        _cancelled = YES;
         if (self.cacheOperation) {
             [self.cacheOperation cancel];
             self.cacheOperation = nil;