Btrfs: Fix memory leak at umount

fs_info, which is allocated in open_ctree(), should be freed
in close_ctree().

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index a5d2249..089871e 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2513,6 +2513,8 @@
 	kfree(fs_info->chunk_root);
 	kfree(fs_info->dev_root);
 	kfree(fs_info->csum_root);
+	kfree(fs_info);
+
 	return 0;
 }