Btrfs: fix use after free when close_ctree frees the orphan_rsv

Near the end of close_ctree, we're calling btrfs_free_block_rsv
to free up the orphan rsv.  The problem is this call updates the
space_info, which has already been freed.

This adds a new __ function that directly calls kfree instead of trying
to update the space infos.

Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b5e3d5f..62cd3b6 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3766,7 +3766,7 @@
 
 	btrfs_free_stripe_hash_table(fs_info);
 
-	btrfs_free_block_rsv(root, root->orphan_block_rsv);
+	__btrfs_free_block_rsv(root->orphan_block_rsv);
 	root->orphan_block_rsv = NULL;
 
 	lock_chunks(root);