Btrfs: Add some simple throttling to wait for data=ordered and snapshot deletion

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index b55dba58..f0fb197 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -345,6 +345,7 @@
 	struct completion kobj_unregister;
 	int do_barriers;
 	int closing;
+	unsigned long throttles;
 
 	u64 total_pinned;
 	spinlock_t delalloc_lock;