Btrfs: fix uninitialized sync_pending

sync_pending is uninitialized before it be used, fix it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 46f9a20..f2a4cc7 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -142,7 +142,7 @@
 	unsigned long limit;
 	unsigned long last_waited = 0;
 	int force_reg = 0;
-	int sync_pending;
+	int sync_pending = 0;
 	struct blk_plug plug;
 
 	/*