Btrfs: fix uninitialized sync_pending

sync_pending is uninitialized before it be used, fix it.
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -142,7 +142,7 @@ static noinline int run_scheduled_bios(struct btrfs_device *device)
 	unsigned long limit;
 	unsigned long last_waited = 0;
 	int force_reg = 0;
-	int sync_pending;
+	int sync_pending = 0;
 	struct blk_plug plug;
 
 	/*