f2fs: use atomic variable for total_extent_tree

It would be better to use atomic variable for total_extent_tree.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 19beabe..a7f6191 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -762,7 +762,7 @@
 	struct rw_semaphore extent_tree_lock;	/* locking extent radix tree */
 	struct list_head extent_list;		/* lru list for shrinker */
 	spinlock_t extent_lock;			/* locking extent lru list */
-	int total_ext_tree;			/* extent tree count */
+	atomic_t total_ext_tree;		/* extent tree count */
 	atomic_t total_ext_node;		/* extent info count */
 
 	/* basic filesystem units */