Btrfs: kill root from btrfs_is_free_space_inode

Since root can be fetched via BTRFS_I macro directly, we can save an args
for btrfs_is_free_space_inode().

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 5e552f9..d1ebd2a 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4444,7 +4444,7 @@
 	int ret;
 
 	/* Need to be holding the i_mutex here if we aren't free space cache */
-	if (btrfs_is_free_space_inode(root, inode))
+	if (btrfs_is_free_space_inode(inode))
 		flush = 0;
 
 	if (flush && btrfs_transaction_in_commit(root->fs_info))