Btrfs: fix wrong size for the reservation of the, snapshot creation

We should insert/update 6 items(root ref, root backref, dir item, dir index,
root item and parent inode) when creating a snapshot, not 5 items, fix it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index e6934de..c56daa3 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -526,7 +526,7 @@
 		*inherit = NULL;	/* take responsibility to free it */
 	}
 
-	trans = btrfs_start_transaction(root->fs_info->extent_root, 5);
+	trans = btrfs_start_transaction(root->fs_info->extent_root, 6);
 	if (IS_ERR(trans)) {
 		ret = PTR_ERR(trans);
 		goto fail;