Btrfs: use the transactions block_rsv for the csum root

The alloc warnings everybody has been seeing is because we have been reserving
space for csums, but we weren't actually using that space.  So make
get_block_rsv() return the trans->block_rsv if we're modifying the csum root.
Also set the trans->block_rsv to NULL so that if we modify the csum root when
running delayed ref's that comes out of the global reserve like it's supposed
to.  With this patch I'm not seeing those alloc warnings anymore.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/extent-tree.c

@@ -3565,10 +3565,12 @@ out:
 static struct btrfs_block_rsv *get_block_rsv(struct btrfs_trans_handle *trans,
 					     struct btrfs_root *root)
 {
-	struct btrfs_block_rsv *block_rsv;
-	if (root->ref_cows)
+	struct btrfs_block_rsv *block_rsv = NULL;
+
+	if (root->ref_cows || root == root->fs_info->csum_root)
 		block_rsv = trans->block_rsv;
-	else
+
+	if (!block_rsv)
 		block_rsv = root->block_rsv;
 
 	if (!block_rsv)
@@ -3865,12 +3867,13 @@ static void release_global_block_rsv(struct btrfs_fs_info *fs_info)
 void btrfs_trans_release_metadata(struct btrfs_trans_handle *trans,
 				  struct btrfs_root *root)
 {
+	struct btrfs_block_rsv *block_rsv;
+
 	if (!trans->bytes_reserved)
 		return;
 
-	BUG_ON(trans->block_rsv != &root->fs_info->trans_block_rsv);
-	btrfs_block_rsv_release(root, trans->block_rsv,
-				trans->bytes_reserved);
+	block_rsv = &root->fs_info->trans_block_rsv;
+	btrfs_block_rsv_release(root, block_rsv, trans->bytes_reserved);
 	trans->bytes_reserved = 0;
 }
 

fs/btrfs/transaction.c

@@ -453,6 +453,7 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 		return 0;
 	}
 
+	trans->block_rsv = NULL;
 	while (count < 4) {
 		unsigned long cur = trans->delayed_ref_updates;
 		trans->delayed_ref_updates = 0;