Btrfs: introduce grab/put functions for the root of the fs/file tree

The grab/put funtions will be used in the next patch, which need grab
the root object and ensure it is not freed. We use reference counter
instead of the srcu lock is to aovid blocking the memory reclaim task,
which invokes synchronize_srcu().

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ctree.h

@@ -1746,6 +1746,7 @@ struct btrfs_root {
 	int force_cow;
 
 	spinlock_t root_item_lock;
+	atomic_t refs;
 };
 
 struct btrfs_ioctl_defrag_range_args {