Btrfs: Fix memory leak on finding existing super

We missed a memory deallocation in commit 450ba0ea.

If an existing super block is found at mount and there is no
error condition then the pre-allocated tree_root and fs_info
are no not used and are not freeded.
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/super.c

@@ -654,6 +654,8 @@ static int btrfs_get_sb(struct file_system_type *fs_type, int flags,
 		}
 
 		btrfs_close_devices(fs_devices);
+		kfree(fs_info);
+		kfree(tree_root);
 	} else {
 		char b[BDEVNAME_SIZE];