Btrfs: do not BUG_ON on aborted situation

Btrfs balance can easily hit BUG_ON in these places, but we want
to it bail out gracefully after we force the whole filesystem to
readonly.  So we use btrfs_std_error hook in place of BUG_ON.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/relocation.c

@@ -3771,7 +3771,11 @@ static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
 	while (1) {
 		progress++;
 		trans = btrfs_start_transaction(rc->extent_root, 0);
-		BUG_ON(IS_ERR(trans));
+		if (IS_ERR(trans)) {
+			err = PTR_ERR(trans);
+			trans = NULL;
+			break;
+		}
 restart:
 		if (update_backref_cache(trans, &rc->backref_cache)) {
 			btrfs_end_transaction(trans, rc->extent_root);

fs/btrfs/volumes.c

@@ -2379,7 +2379,11 @@ static int btrfs_relocate_chunk(struct btrfs_root *root,
 		return ret;
 
 	trans = btrfs_start_transaction(root, 0);
-	BUG_ON(IS_ERR(trans));
+	if (IS_ERR(trans)) {
+		ret = PTR_ERR(trans);
+		btrfs_std_error(root->fs_info, ret);
+		return ret;
+	}
 
 	lock_chunks(root);
 
@@ -3050,7 +3054,8 @@ static void __cancel_balance(struct btrfs_fs_info *fs_info)
 
 	unset_balance_control(fs_info);
 	ret = del_balance_item(fs_info->tree_root);
-	BUG_ON(ret);
+	if (ret)
+		btrfs_std_error(fs_info, ret);
 
 	atomic_set(&fs_info->mutually_exclusive_operation_running, 0);
 }